add osmo_sockaddr_str API

For handling RTP IP addresses and ports, osmo-mgw, osmo-bsc and osmo-msc
so far have their own separate shims and code duplication around
inet_ntoa(), htons(), sockaddr conversions etc. Unify and standardize
with this common API.

In the MGW endpoint FSM that was introduced in osmo-bsc and which I
would like to re-use for osmo-msc (upcoming patch moving that to
osmo-mgw), it has turned out that using char* IP address and uint16_t
port number types are a convenient common denominator for logging,
MGCP message composition and GSM48. Ongoing osmo-msc work also uses this
for MNCC.

This is of course potentially useful for any other IP+port combinations
besides RTP stream handling.

Needless to say that most current implementations will probably stay
with their current own conversion code for a long time; for current
osmo-{bsc,msc,mgw} work (MGW endpoint FSM) though, I would like to move
to this API here.

Change-Id: Id617265337f09dfb6ddfe111ef5e578cd3dc9f63
diff --git a/src/Makefile.am b/src/Makefile.am
index 27ab702..1fae8b0 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -24,7 +24,9 @@
 			 macaddr.c stat_item.c stats.c stats_statsd.c prim.c \
 			 conv_acc.c conv_acc_generic.c sercomm.c prbs.c \
 			 isdnhdlc.c \
-			 tdef.c
+			 tdef.c \
+			 sockaddr_str.c \
+			 $(NULL)
 
 if HAVE_SSSE3
 libosmocore_la_SOURCES += conv_acc_sse.c