dial by name in chan_dahdi

* chan_dahdi supports dialing configuring and dialing by device file name.
  DAHDI/span-name!local!1 will use /dev/dahdi/span-name/local/1 . Likewise
  it may appear in chan_dahdi.conf as 'channel => span-name!local!1'.
* A new options for chan_dahdi.conf: 'ignore_failed_channels'. Boolean.
  False by default. If set, chan_dahdi will ignore failed 'channel' entries.
  Handy for the above name-based syntax as it does not depend on
  initialization order.
* have my_pri_make_cc_dialstring() only manupulate dial-strings of group
  (gGrR) dialing, which make it lsightly more complicated.

https://reviewboard.asterisk.org/r/535/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@269238 65c4cc65-6c06-0410-ace0-fbb531ad65f3