miext: Move SyncShm FDs out of the way of clients

miext: Move SyncShm FDs out of the way of clients

Applications may end up allocating a bunch of shmfence objects, each
of which uses a file descriptor, which must be kept open lest some
other client ask for a copy of it later on.

Lacking an API that can turn a memory mapping back into a file
descriptor, about the best we can do is push the file descriptors out
of the way of other X clients so that we don't run out of the ability
to accept new connections.

This uses fcntl F_GETFD to push the FD up above MAXCLIENTS.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>