plugins: Enable message_waiting in ril

Enable the message_waiting atom in the ril plugin.
This exposes the MessageWaiting interface which will
allow notification when new voicemails arrive.
rilmodem: fix decline call behavior (LP: #1260988)

When a remote call with STATUS_INCOMING is declined,
RIL_REQUEST_HANGUP_WAITING_OR_BACKGROUND should be
called instead of RIL_REQUEST_HANGUP, as some operators
may not forward the caller to voicemail.
src: fix sms_mwi_dcs_decode() return param bug

The return parameter 'active' was being incorrectly set to
an int value instead of a boolean value. This caused the
message-waiting code to break.
Merge pull request #47 from tonyespy/message-waiting

plugins: Enable message_waiting in ril
Merge pull request #48 from tonyespy/lp1260988

Lp1260988 - Decline call voicemail fix