Add API to register callback to set socket options

The registered callback function will be called before connect() is
called on the socket, allowing tweaking of much more than just keepalive
settings.

This deprecates xmpp_conn_set_keepalive().

Includes helper/example callback function to set default keepalive
parameters that some library users may find useful.

ChangeLog

@@ -3,6 +3,8 @@
 	- Add code coverage support
 	- New API:
 		- xmpp_stanza_get_child_by_path()
+		- xmpp_conn_set_sockopt_callback()
+		- xmpp_sockopt_cb_keepalive()
 
 0.11.0
 	- SASL EXTERNAL support (XEP-0178)