driver: factor out driver_send_reply()
driver: factor out driver_send_reply()

This is not a functional change, just moves some code into its own
function. This is analogous to driver_send_error() but relies on the
header and body to have already been serialized.

Signed-off-by: Tom Gundersen <7c8224529a7c718e876cb2496c973509fde1e50e@jklm.no>

bus1/dbus-broker