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/tests/testsuite.at b/tests/testsuite.at
index 0093403..db2003f 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -331,3 +331,9 @@
 cat $abs_srcdir/tdef/tdef_test.ok > expout
 AT_CHECK([$abs_top_builddir/tests/tdef/tdef_test], [0], [expout], [ignore])
 AT_CLEANUP
+
+AT_SETUP([sockaddr_str])
+AT_KEYWORDS([sockaddr_str])
+cat $abs_srcdir/sockaddr_str/sockaddr_str_test.ok > expout
+AT_CHECK([$abs_top_builddir/tests/sockaddr_str/sockaddr_str_test], [0], [expout], [ignore])
+AT_CLEANUP