Changes

ggsn/ggsn_vty: create state-dir

Prepare to change the state-dir in the default config in a follow-up
commit. Create the directory if it does not exist.

Change-Id: I91349fb284336a9de6af41475f1b824eb0e021b0

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

contrib/systemd: run as osmocom user

Related: OS#4107
Change-Id: I915f2fc12d0bd905d24636aacb2760a6b72a55e3

Use uniform log format for default config files

Related: OS#6272
Change-Id: I272767e029e95b64f2525d4f19efdfa1f0e29ca2

{contrib,debian}/osmo-ggsn.init: remove

Remove SysV init scripts. These are not really maintained anymore and
this makes it consistent with other Osmocom projects.

Avoids synchronizing with SysV scripts on debian:
  # systemctl enable osmo-ggsn
  Synchronizing state of osmo-ggsn.service with SysV service script with /lib/systemd/systemd-sysv-install.
  Executing: /lib/systemd/systemd-sysv-install enable osmo-ggsn

Change-Id: I11bfb5122344b54970b7f742470cb74b95fa37e0