Enable PRI SERVICE message support in chan_dahdi for the 'national' switchtype

Enable PRI SERVICE message support in chan_dahdi for the 'national' switchtype

Revision 1072 of libpri added SERVICE message support for the 'national'
switchtype. The attached patch enables the use of 'pri service' CLI commands
on dahdi channels that are configured for the 'national' switchtype.

(closes issue #17142)
Reported by: dhubbard
Patches:
      dw-ni2.patch uploaded by dhubbard (license 733)
Tested by: elguero, dhubbard

Review: https://reviewboard.asterisk.org/r/612/

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@257642 65c4cc65-6c06-0410-ace0-fbb531ad65f3