Fixed channel selection on outgoing calls on NT-mode multipoint

On a multipoint bus, it is required to assign a channel with no other
alternative allowed. This is required, because an individual phone on the
bus may not choose a different channel, while other phones accept the
indicated channel. Also an individual phone does not have the information
about other available channels. On a point-to-point configuration it is
possible anyway.

In this multipoint case, the channel assignment is now forced automatically,
even if the 'force' keyword is not specified in the "out-channel" of
interface.conf.

Additionally "lcradmin portinfo" shows the channel selection settings now.