Bug 980701 - Make sure we do not try to place more than two calls

We cannot open more than two lines, and mozTelephony is not able yet to
tell us that we are doing this. So we do a check ourself for now, and
bug 990472 should fix it at mozTelephony level.
Merge pull request #17982 from lissyx/bug980701

Bug 980701 - Make sure we do not try to place more than two calls r=etienne