Add support for FreeRDP 3.x

I really don't know much about it, so I went grep'ing around to see how things changed between 2.x and 3.x and adjusted accordingly. So far it seems to work...

The main source of changes is the settings are now opaque and require the use of accessors. That was pretty mechanical.

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.

Note that the screen size is odd (and different between freerdp client and remmina), it also won't adjust dynamically when the window is resized. I don't think this relates to my port though, I observe the same behaviour with the packaged FreeRDP 2 based Weston, but I can try to look into it later

Signed-off-by: Benjamin Herrenschmidt benh@kernel.crashing.org