[rilmodem][WIP] support for hidden reset

Hidden reset here means storing the PIN and givin it to modem without
user knowledge if modem reboots without power down. It also means
bringing up the interfaces back.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #55 from jkangas/master

Request for pulling hidden reset support to main branch