introduce import-event-options in xml event definitions

Related to rhbz#990285

Signed-off-by: Jakub Filak <jfilak@redhat.com>
introduce 'watch_Bugzilla' event

This event adds user's bugzilla login to CC List of a bug.
This event expects that 'reported_to' element contains at least one
'Bugzilla: URL=' record. In case of several bugzilla records uses the
last one.

The event's XML definition imports configuration of 'report_Bugzilla'
event.

Related to rhbz#990285

Signed-off-by: Jakub Filak <jfilak@redhat.com>
spec: add watch_Bugzilla.xml

Related to rhbz#990285

Signed-off-by: Jakub Filak <jfilak@redhat.com>
client API: return empty string instead of NULL

run_event machine can't handle NULLs returned when client runs in
noninteractive mode. run_event requires empty strings which denotes
"empty answer"

Related to rhbz#990285

Signed-off-by: Jakub Filak <jfilak@redhat.com>