screen-share: Allow fullscreen shell command to be configured

screen-share: Allow fullscreen shell command to be configured

I've updated this based on comments, simplifying the command handling.

Currently the screen-share module uses a hard-coded command to start the
fullscreen shell server. This patch causes the module to read the command from
the weston config file (from the "command" key in the "screen-share" section).
The default value remains the same (i.e. to run weston with the RDP backend and
fullscreen shell), but is now located in the weston config file.

As well as allowing the arguments to the fullscreen shell server to be changed,
this also permits an alternative fullscreen shell server to be used if required,
without needing to recompile. Since the command is run as the user running
weston, this should not pose any additional security risk.

Signed-off-by: Andrew Wedgbury <andrew.wedgbury@realvnc.com>