Support SIP line selection for outgoing call

For one voice port, multiple SIP lines (SIP providers) can be
configured. By dialing line selection prefix before calling number,
one of the lines is selected to make the outgoing call.

The line selection prefix is configured via UCI
"asterisk.<line>.selector".

For example, if setting UCI "asterisk.line1.selector='11'",
then dialing '11' will select line1 to make the outgoing call.