res_pjsip: Add an 'auto' option for DTMF Mode

This patch adds support for automatically detecting the type of DTMF that a
PJSIP endpoint supports. When the 'dtmf_mode' endpoint option is set to 'auto',
the channel created for an endpoint will attempt to determine if RFC 4733
DTMF is supported. If so, it will use that DTMF type. If not, the DTMF type
for the channel will be set to inband.

Review: https://reviewboard.asterisk.org/r/4438

ASTERISK-24706 #close
Reported by: yaron nahum
patches:
  yaron_patch_3_Feb.diff submitted by yaron nahum (License 6676)
........

Merged revisions 434637 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434638 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_pjsip.c

@@ -210,6 +210,10 @@
 							<enum name="info">
 								<para>DTMF is sent as SIP INFO packets.</para>
 							</enum>
+                                                        <enum name="auto">
+                                                                <para>DTMF is sent as RFC 4733 if the other side supports it or as INBAND if not.</para>
+                                                        </enum>
+
 						</enumlist>
 					</description>
 				</configOption>