osmo-gsm-manuals: update GSUP docs
GSUP used to be called "GPRS Subscriber Update Protocol" and its use was limited to the SGSN.
However, GSUP has since made it to libosmocore and was recoined as "Generic Subscriber Update Protocol".
It will be used by OsmoNITB / OsmoMSC.
- rename to 'Generic ...'
- as client, don't assume it is always the SGSN
When the VLR #1592 is implemented: Possibly put these chapters elsewhere and/or also include in the
OsmoNITB OsmoMSC manual.