The native error numbers (from <errno.h>) and our nmerr extention on top
of them are almost the same. But there are peculiarities.

Both errno and nmerr must be positive values. That is because some API
(systemd) like to return negative error codes. So, a positive errno and
its negative counter part indicate the same error. We need normalization
functions that make an error number positive (these are nm_errno() and
nm_errno_native()).

This means, G_MININT needs special treatment, because it cannot be
represented as a positive integer. Also, zero needs special
treatment, because we want to encode an error, and zero already encodes
no-error. Take care of these special cases.

On top of that, nmerr reserves a range within native error numbers for
NetworkManager specific failure codes. So we need to transition from native
numbers to nmerr numbers via nm_errno_from_native().

Take better care of some special cases and clean them up.

Also add NM_ERRNO_NATIVE() macro. While nm_errno_native() coerces a
value in the suitable range, NM_ERRNO_NATIVE() asserts that the number
is already positive (and returns it as-is). It's use is only for
asserting and implicitly documenting the requirements we have on the
number passed to it.