Split gprs_sgsn.{c,h} -> {auth,mmctx,sgsn}.{c,h}

Some level of split already existed, like sgsn_auth.c, but headers were
entangled together.
Let's clearly separate application centric code (sgsn.c/h), auth related
code (auth.c/h) and mmctx related code (mmctx.c/h).

Change-Id: I048a082851c1275c959649942904205b02acce2a
diff --git a/src/sgsn/gprs_gmm.c b/src/sgsn/gprs_gmm.c
index 2f15c92..5bf66c3 100644
--- a/src/sgsn/gprs_gmm.c
+++ b/src/sgsn/gprs_gmm.c
@@ -48,7 +48,7 @@
 
 #include <osmocom/sgsn/debug.h>
 #include <osmocom/sgsn/gprs_llc.h>
-#include <osmocom/sgsn/gprs_sgsn.h>
+#include <osmocom/sgsn/mmctx.h>
 #include <osmocom/sgsn/gprs_gmm.h>
 #include <osmocom/sgsn/gprs_utils.h>
 #include <osmocom/sgsn/gprs_subscriber.h>