gsn_restart is created in CWD
Similar to osmo-msc issue #4735, cwd is / when run from systemd, so we get gsn_restart file created in the system root.
VTY: Add gtp state-dir command
The SGSN initialises GTP with gtp_statedir of "./" which may
not be the desired path for writing the gsn_restart file.
When starting from systemd for example, we might write
to the system root.
This patch allows override via the config file.