Store keepalive parameters in conn object. This allows call the function
with a disconnected conn. Also, the parameters are not cleared on
reconnect and applied once a socket is created.
xmpp_conn_set_flags() and xmpp_conn_get_flags() unify interface of
connection configuration.
This interface allows compile-time check of supported features or even
run-time check. Therefore, applications can be built with older
libstrophe which doesn't support some optional flags.
Having debugging on by default in the example seems to help
people trying out the library, and with the default_logger()
call it's not too much extra complication.
We should also write an example/complex.c that exercises
all the callback options, a custom allocator and so on.
