gtphub: add first bits of GRX ares

For the resolving function, change the function signature to return a
gtphub_peer_port. In consequence, publish two functions concerned with
gtphub_peer_port instances for use in test and gtphub_ext.c.

Add GGSN resolution queue, callback and cache. Simple implementation: if an
SGSN asks for a GGSN, it will first get no answer, and I hope it will ask again
once the GGSN is in the cache.

Within gtphub_ext.c, have a dummy sgsn struct, as the sgsn_ares code currently
depends on it (half the functions pass an sgsn instance pointer around, but the
other half use the global one).

In the unit tests, wrap away the ares initialization so that they can work
without a DNS server around. The netcat test breaks because of this, will
remove it.

Using sgsn_ares, implement the gtphub_resolve_ggsn_addr() function, I hope:
untested.

Minor cosmetics just to see if you're paying attention... ;)

Sponsored-by: On-Waves ehi
diff --git a/openbsc/src/gprs/gprs_utils.c b/openbsc/src/gprs/gprs_utils.c
index 2293f02..ad479db 100644
--- a/openbsc/src/gprs/gprs_utils.c
+++ b/openbsc/src/gprs/gprs_utils.c
@@ -114,7 +114,9 @@
 }
 
 /* TODO: Move these conversion functions to a utils file. */
-/**
+/* TODO: consolidate with gprs_apn2str(). */
+/** memmove apn_enc to out_str, replacing the length octets in apn_enc with '.'
+ * (omitting the first one) and terminating with a '\0'.
  * out_str needs to have rest_chars amount of bytes or 1 whatever is bigger.
  */
 char * gprs_apn_to_str(char *out_str, const uint8_t *apn_enc, size_t rest_chars)