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Prior to this change, if waypipe was not built with dmabuf replication
supported, then the dmabuf portion of the headless test would still run
and fail.

Also use weston-simple-dmabuf-egl as a test application instead of
weston-simple-dmabuf-drm; the latter has been dropped from Weston.

Change location of test bit for endianness detection to something that is
guaranteed not to be set if the byte order is wrong.

When the bit to check endianness is not set, there are two possibilities:
a) the endianness of the remote system actually differs
b) a generally bad connection token, e.g. from older Waypipe versions

... by specifying what the minimum Waypipe version on both sides
should be. 0.7.0 has not yet been released, but will be the next
version number.

In case it’s impossible to install waypipe system-wide, or if it is
named something else than “waypipe”, it can be useful to set the name or
path of the remote binary.

Also check for EAGAIN, in case EAGAIN != EWOULDBLOCK.

It is possible for poll to indicate POLLHUP and still have outstanding
data to be read. To ensure that this data is read, the main loop code
now treats POLLHUP as POLLIN, and only acts on the hang-up when the
socket being read from actually returns EOF/zero bytes read.

This makes it easier to check when two versions of waypipe built from
a git repository are based on the same commit. This does not take into
account any build flag differences or extra changes to the working tree.

... and ensure it doesn't happen by accident, by simplifying the
corresponding function signature.

Starting the programs used in this test may be slow due to disk reads
and context switches, especially with overloaded systems/empty caches.

Reading the header (including the version field) separately from
the key field ensures that Waypipe correctly prints an error when
an older version of Waypipe (that used an 8-byte instead of a
16-byte connection token) tries to connect.

Tag each Wayland message sent over the wire with the number of file descriptors
that Waypipe's logic has associated with the message. This tag makes it easier
to detect if the Waypipe client and server instances, for some reason, have
interpreted the two streams of file descriptors differently. (For example, this
may be useful if one instance supports a new Wayland protocol involving a file
descriptor, but the other does not.)

This change introduces a limit of 31 fds per method and 2047 different methods
per Wayland protocol type. It is very unlikely that a well written protocol will
ever surpass either of these two limits.

This change:
* Expands the connection token used by waypipe client/server to use 16 bytes,
  and dedicates 4 of those bytes for versioning and flags.
* Adds a version number, so that in case we need to break wire protocol,
  we can do so and provide clear error messages.
* Adds a fixed bit to the connection token to check if the remote endianness
  does not match the local endianness

This was previously calibrated against Weston, but it turns out that
Weston was wrong and had used clockwise rotations where the Wayland
specification required counterclockwise rotations.

According to the Python documentation, the new usage should be valid
for Python >=3.3, covering a wider range than the >=3.5 for
subprocess.Popen. Also, this change avoids a possible regression
with Python 3.8.0 in which the TimeoutExpired exception no longer
provides a non-None .output field.

When queuing a buffer diff construction task on a DMABUF, the
buffer is mapped; if a buffer diff application message is
received before the task completes, the buffer is mapped and
unmapped a second time, making sfd->mem_local NULL. This change
ensures that diff and fill message handlers do not blindly
remap DMABUFs.

On some systems with gcc, a (void)cast does not suffice to ignore
the return values of read() and write(). However, in some cases,
such as when closing Waypipe via SIGINT, the program will still
shut down no matter the return value of the write() used to indicate
that a SIGINT occured.

This avoids unnecessarily creating a socketpair that would never
be used, and ensures that if no reconnection control pipe is set,
a killed connection will never leave the client side alive and
unrecoverable.

(The downside of this change is that the number of combinations
of behaviors has gotten even larger.)

Specifically, remove the -C compression option from the example wrapped
ssh command, since it is often faster to use e.g. `waypipe -c lz4 ssh`
instead.

That most dependencies are optional has been clarified by moving them into
a separate list.

The `cd path/..` idiom is not as clear as two distinct cd commands.

This ensures a) that it is less likely for applications to see
inconsistent state during shutdown, and b) that the messages used
to indicate that the remote program/compositor shut down, actually
are sent after the most recently queued message is sent, and not
injected at some random byte position.

Depending on how messages are grouped, it was possible to transfer data
to a pipe shadow structure and then shutdown the pipe before the data
is written. This commit adds a new state flag, 'pending_w_shutdown',
which is used to delay the shutdown.

The main loop code (and some of shadow.c) has been adjusted to
use different error codes for allocation failure and for regular
errors.

If there is an error, then the connection closing message will
indicate the type (oom/error/natural connection close). For the
first two types, a corresponding error event will also be sent to the
Wayland application.