gprs_gmm: introduce a GMM Attach Request FSM

The old GMM Attach Request handling used a recursive function
which can not handle certain states and is quite complex and hard to
extend.

The new FSM handles such request in a FSM and can be called multiple
times.

Change-Id: I58b9c17be9776a03bb2a5b21e99135cfefc8c912
diff --git a/src/gprs/Makefile.am b/src/gprs/Makefile.am
index 46d94d4..0f7c5fa 100644
--- a/src/gprs/Makefile.am
+++ b/src/gprs/Makefile.am
@@ -59,6 +59,7 @@
 	$(NULL)
 
 osmo_sgsn_SOURCES = \
+	gprs_gmm_attach.c \
 	gprs_gmm.c \
 	gprs_sgsn.c \
 	gprs_sndcp.c \