Add support for FreeRDP 3.x

With this, Weston can build against either FreeRDP 3.x or 2.x depending
on what has been detected by meson (3.x takes priority).

The main source of changes is the settings are now opaque and require the
use of accessors. That was pretty mechanical and seems to work on 2.x as
well.

There are a few changes around constants getting a WINPR_ prefix, the UTF
conversion functions we used are obsolete, so use the proper "new" ones,
and other fairly minor things.

The key & cert management changed rather completely, libfreerdp won't load
files for us, we have to use the helpers to do so, and I *think* the RDP RSA
key and SSL key use the same setting location. Seems to work with SSL at
least.

There was also a minor glitch with keyboard input, KBD_FLAGS_DOWN is basically
never set. It appears to be an upstream FreeRDP change in 3.x, it was being
set incorrectly (always on any key down) while it should only be set on
repeats. However the fastpath input code has no way to set it from wh...