compositor: refactor client forking code

shell.c and tablet-shell.c had almost the same code for forking their
special shell client. Generalise this code and put it into
wlsc_client_launch() in compositor.c.

Improve error cleanup and reporting in wlsc_client_launch().

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
4 files changed