ggsn: Split application lifecycle related code into ggsn_main.c

This way we further shrink ggsn.c and leave there GGSN related code.

Change-Id: I9e6a3beac7657f0a8c02d514b54c6f1caa93bba7
diff --git a/ggsn/Makefile.am b/ggsn/Makefile.am
index 022cdef..a8ddf1e 100644
--- a/ggsn/Makefile.am
+++ b/ggsn/Makefile.am
@@ -12,4 +12,4 @@
 endif
 
 osmo_ggsn_DEPENDENCIES = ../gtp/libgtp.la ../lib/libmisc.a
-osmo_ggsn_SOURCES = ggsn_vty.c ggsn.c ggsn.h icmpv6.c icmpv6.h checksum.c checksum.h pco.c pco.h
+osmo_ggsn_SOURCES = ggsn_main.c ggsn_vty.c ggsn.c ggsn.h icmpv6.c icmpv6.h checksum.c checksum.h pco.c pco.h