PMM Connected defines a Iu signaling connection. The 2 other
PMM states do not have an active Iu signaling connection.
Change-Id: Ie05d2bdf8dfb593b4c7e837107a3a06f22e90119
RANAP related functionalities were splitted among several files
(gprs_gmm.c, gprs_sgsn.c and sgsn_libgtp.c). Let's move it into its own
file to shrink complexity/size of existing files.
It also allows to keep a lot of conditionally enabled code (BUILD_IU)
and its dependencies (osmo-iuh) together.
Change-Id: I549042aaff045a378de77d657cc396ee08f22f33