Bug 861725 - B2G Emulator: support call forwarding

Please see [bug 861725](https://bugzilla.mozilla.org/show_bug.cgi?id=861725).

This PR implements AT+CCFC command [1].

[1] TS 27.007 session 7.11
`%d,%d%*s` because the second comma is omitted if no further command parameter is given.