libmsc: move L3 call-control to separate C file (gsm_04_08_cc.c)

The CC sub-layer is fairly self-contained, so let's move it to
a separate C source file.  The old gsm_04_08.c file now only
contains the 04.07 / DTAP core and MM sub-layer handling.

I did this initially as an experiment to see how self-contained
our CC implementation really is.  Given this rather straight-forward
patch builds fine, CC really is self-contained (yay!).

Change-Id: Idb8dd7a8d9d8b4a28c492f12da3cc3305b695cca
4 files changed