[bluez] Fix call hold handling. Contributes to JB#10677, fixes JB#10670.

Call hold indicator was never zeroed; fixed to set indicator on every
call status change. As there is often a sequence of status changes
resulting from a user action e.g. when an active call is disconnected
and a held call becomes active, there is also a corresponding sequence
of indicator changes instead of just one change.

Index errors were not reported for CHLD=1x or CHLD=2x commands.

Give errors to unsupported CHLD commands (if support for enhanced
call control or three-way handling is configured off)

Supplementary configuration key for turning off conference calls
while still supporting other three-way calling features.