pjsip: allow and disallow show same codecs

In order to prevent confusion over the allow and disallow
list of codecs being the same an option for registering a
field as an alias is added.  The alias field will be read
from the configuration file, but afterwards is not listed
as a known field.  With disallow set as an alias, the CLI
command pjsip show endpoint # will list the allow= field,
but not the disallow field.

(closes issue ASTERISK-23092)
Review: https://reviewboard.asterisk.org/r/3193/
........

Merged revisions 410190 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410191 65c4cc65-6c06-0410-ace0-fbb531ad65f3