socket.h: Introduce API osmo_sockaddr_netmask_to_prefixlen()

Implementation is imported from osmo-ggsn.git
97f60e3dca581797007524e0006ca9fafad59713 in46a_netmasklen() and adapter
to work with an osmo_sockaddr.

This will be used by osmocom-bb's "modem" app.

Change-Id: I75e75e251c6776801fffdde745aebedf21c68799
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 6ac8db8..86363e1 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -7,5 +7,5 @@
 # If any interfaces have been added since the last public release: c:r:a + 1.
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what			description / commit summary line
-libosmocore new API osmo_sockaddr_is_any()
+libosmocore new API osmo_sockaddr_is_any(), osmo_sockaddr_netmask_to_prefixlen()
 libosmocore	ABI breakage		OSMO_NUM_DLIB change affecting internal_cat[]