Changes

GMM: rework PDP context status

Use an uint16 instead of an pointer with an unknown length.
Also encode the current active PDP context (e.g. on 2G/4G
mobilty this could change). It resynchronizes the UE with the SGSN
state.

Change-Id: I12d3110c9365132f96ef7ff8a1be22a431682c81

Refactor mmctx_is_r99 into mmctx.c

It is related to the mmctx and should be in mmctx.c instead
of the GMM message parsing or handling.

Change-Id: I198a3c9d49667c70e8995808b3fcb5ea26e8f17a

gprs_ms_net_cap_gea_mask(): protect against empty MS Network Capabilities

When the MS Network Capability is empty, expect to support at least
unencrypted communications.
This shouldn't be empty at all, as the minimum length of it is 2 when
the MS/UE is doing an Attach Request.
But when receiving a MS/UE SGSN Context via Gn, the field
is optional.
The MS/UE shall include it into the Routing Area Request,
but not must.

Change-Id: Ieef2e3eeaaadc90c35fff6f20d47bd36aaa4b9e6

Add include prefix for all libgtp related includes

The libgtp includes contain all the prefix osmocom/gtp.
The old non-prefix was used in the old OpenGGSN code base.

Change-Id: I5ea50c9766f719e1eea89cbf6783fafc2093773d