Merge pull request #55 from jkangas/master

Request for pulling hidden reset support to main branch
[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>