Add support for Linux namespaces

Add support for Linux namespaces

If compiled with --enable-namespaces, dgamelaunch will now create new
user and network namespaces before chrooting.  This means that it
works without root (if unprivileged user namespaces are enabled) and
cannot access the network.

Without namespaces:
-chroot
-setgroups/setgid/setuid to shed_gid/shed_uid

With namespaces:
-unshare to create a new user and network namespace
-write to /proc/self/{uid,gid}_map to set uid/gid to shed_uid/shed_gid
-chroot
-drop all capabilities