Changes

doc: set state-dir to /var/lib/osmocom/osmo-ggsn

Prepare to run osmo-ggsn as user with the systemd service. As with other
Osmocom service files, we will set StateDirectory= and WorkingDirectory=
options. This results in osmo-ggsn only being able to write to
/var/lib/osmocom, therefore let's change the state-dir from /tmp to
/var/lib/osmocom/osmo-ggsn to avoid:

  gsn.c:411 fopen(path=/tmp/gsn_restart, mode=w) failed: Error = Permission denied

Having the state in /var/lib/osmocom also makes more sense, because then
it doesn't get deleted on reboot.

Change-Id: I5b51529b4f8bd2462e54f58a1ce2e2d7c76ff46a