mncc: rework passing GCR over the MNCC interface

Using *unpacked* 'struct osmo_gcr_parsed' in the MNCC PDUs makes
the protocol even more complicated than it currently is, and
moreover complicates implementing MNCCv8 in the ttcn3-sip-test.

Replace 'struct osmo_gcr_parsed' in 'struct gsm_mncc' with a
fixed-length buffer, which is supposed to hold the Global Call
Reference encoded as per 3GPP TS 29.205.

Indicate presence of GCR using the MNCC_F_GCR flag.

Change-Id: I259b6d7e4cbe26159b9b496356fc7c1c27d54521
Fixes: I705c860e51637b4537cad65a330ecbaaca96dd5b
Related: OS#5164, OS#5282
diff --git a/tests/mncc/mncc_test.ok b/tests/mncc/mncc_test.ok
index 7cb1201..059e808 100644
--- a/tests/mncc/mncc_test.ok
+++ b/tests/mncc/mncc_test.ok
@@ -1,15 +1,15 @@
 test_sdp_termination()
 
 struct gsm_mncc:
-empty SDP: len=1872 sdplen=1024 sdp="\0" rc=0
-empty SDP, shortest possible: len=849 sdplen=1 sdp="\0" rc=0
-empty SDP, zero len: len=848 sdplen=0 sdp=- rc=-22
-terminated SDP str: len=1872 sdplen=1024 sdp="Privacy is a desirable marketing option\0" rc=0
-terminated SDP str, shortest possible: len=888 sdplen=40 sdp="Privacy is a desirable marketing option\0" rc=0
-terminated SDP str, but len excludes nul: len=887 sdplen=39 sdp="Privacy is a desirable marketing option" rc=-22
-terminated SDP str, but len too short: len=871 sdplen=23 sdp="Privacy is a desirable " rc=-22
-len way too short: len=10 sdplen=-838 sdp=- rc=-22
-len zero: len=0 sdplen=-848 sdp=- rc=-22
+empty SDP: len=1876 sdplen=1026 sdp="\0" rc=0
+empty SDP, shortest possible: len=851 sdplen=1 sdp="\0" rc=0
+empty SDP, zero len: len=850 sdplen=0 sdp=- rc=-22
+terminated SDP str: len=1876 sdplen=1026 sdp="Privacy is a desirable marketing option\0" rc=0
+terminated SDP str, shortest possible: len=890 sdplen=40 sdp="Privacy is a desirable marketing option\0" rc=0
+terminated SDP str, but len excludes nul: len=889 sdplen=39 sdp="Privacy is a desirable marketing option" rc=-22
+terminated SDP str, but len too short: len=873 sdplen=23 sdp="Privacy is a desirable " rc=-22
+len way too short: len=10 sdplen=-840 sdp=- rc=-22
+len zero: len=0 sdplen=-850 sdp=- rc=-22
 
 struct gsm_mncc_rtp:
 empty SDP: len=1168 sdplen=1024 sdp="\0" rc=0