xwayland: Don't use abstract socket on non-Linux systems

Abstract socket is a Linux-only feature. On operating systems other than
Linux, we can only use the normal UNIX socket.

This patch allows mutter to run as a nested Wayland compositor under
Xorg on FreeBSD.

!954
Status Job ID Name Coverage
  Review
passed #518011
check-commit-log

00:00:22

 
  Build
passed #518012
build-mutter

00:02:19

passed #518013
build-without-native-backend-and-wayland

00:02:03

 
  Test
passed #518015
can-build-gnome-shell

00:01:14

passed #518014
test-mutter

00:02:41