features.c: Make Bridge application tolerate unspecified channel.

The Bridge application was inconsistent if the channel to bridge with is
not specified.  If no parameters are given then a warning is issued and
the current channel is hung up.  If options are given but no channel is
specified then a warning is issued and the current channel is not hung up.

* Made the Bridge application give a verbose message instead of a warning
if the channel to bridge with is not specified and made not hang up the
current channel.  As a result dialplan no longer needs to check if a
channel name is passed before calling Bridge and simply needs to check the
BRIDGERESULT channel variable instead.  This is something you likely want
your dialplan to do anyway.

* Fixed up L() option warning message.  It is up to the caller to
determine if the channel is hung up because of the warning.  Dial() hangs
up the current channel while Bridge() does not.

Change-Id: I44349a8dc3912397f28852777de04f19e7bb9c73