add vars: flip_record_on_hold to make the recording flip to the other leg on hold, record_check_bridge to make recording the same file on the opposite leg of a bridge considered a duplicate attempt and record_toggle_on_repeat to make repeat recording the same file toggle the recording off

add code to pass recording bugs on to other legs when executing an attended transfer, needs testing and possible follup commits before using

ZT_EVENT_ABORT and ZT_EVENT_OVERRUN are HDLC events.
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>

this is needed to differentiate route/cfwd