[rilmodem][WIP] support for release_all_held

Telepathy-ring seems to execute "hold and answer" and "hangup and answer"
by calling HoldAndAnswer both cases and also hangup in the later case. In
order to make that work rilmodem driver needs support for release_all_held.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #56 from jkangas/master

Request for pulling support for release_all_held to main branch