/var/lib/gdm is the gdm user's homedir; it's required to be writable for
the gdm session.

/var/lib/xkb is needed for switching keyboard layouts.

Changes to init scripts to better support stateless:

--rc.sysinit--

 - don't update mtab in a r/o root
 - mount on sys and /dev/pts returns EBUSY on a readonly root
 - [ -n $CLIENTSTATE ] is always returning true, causing an unnecessary timeout

--network--

 - /etc/mtab may not have any mount information in the readonly-root
   cause, but the root mount is likely to be a network mount.  Without
   this, nfs hangs the reboot process.

--halt--

 - don't try and update a read-only mtab

--rwtab--

 - /var/empty/sshd/etc/localtime needs to be overwritten by sshd at
   startup
 - /var/lib/random-seed is written to at shutdown; this fixes the
   error but doesn't return the functionality lost in a r/o /var/lib.