change default config filename to osmo-sgsn.cfg, not osmo_sgsn.cfg

All other Osmocom programs I know of have a default config file using a dash.
Comply.

Be backwards compatible: when a legacy osmo_sgsn.cfg exists but no
osmo-sgsn.cfg, use the old config file instead. (Verified to work by manual
tests.)

Change-Id: If804da17a7481e79e000fe40ae0d9c4be9722e61