sbus: Rename sbus_conn_send() methods

Rename sbus_conn_send() -> sbus_conn_send_with_reply() and
sbus_conn_send_reply() -> sbus_conn_send(). This is more inline
with other dbus libraries.

Once we start sending signals using sbus_conn_send_reply() would
not have made sense.

Later commits will refine the sending of reply messages.
be: Remove needless if() check

Simplify flow of function in preparation for next patch.
reply is never NULL at this point.
WIP remove introspection
WIP request
WIP dbus invoke