gb: Add bssgp_msgb_copy function

This function originates from openbsc/src/gprs but is just specific
to BSSGP/Gb on the same level like bssgp_msgb_alloc.

This commit puts the former gprs_msgb_copy function beside
bssgp_msgb_alloc.

Renamed function:

  gprs_msgb_copy -> bssgp_msgb_copy

Sponsored-by: On-Waves ehf
5 files changed