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.
In order to do proper deprecation of those API's we re-introduce them
so they can be removed in the next release. This will then also lead to
an ABI bump.
Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>