Fix / Clean Up Some Items To Handle The New auto_* NAT Options

The original report had to do with a realtime peer behind NAT being pruned and
the peer's private address being used instead of its external address.  Upon
debugging, it was discovered that this was being caused by the addition of
the auto_force_rport and auto_comedia settings.

This patch does the following:

* Adds a missing note to the CHANGES file indicating that the default global nat
  setting is auto_force_rport

* Constify the 'req' parameter for check_via()

* Add calls to check_via() in a couple of places in order for the auto_*
  settings to do their job in attempting to determine if NAT is involved

* Set the flags SIP_NAT_FORCE_RPORT and SIP_PAGE2_SYMMETRICRTP if the auto_*
  settings are in use where it was needed

* Moves the copying of peer flags up in build_peer() to before they are used;
  this fixes the realtime prune issue

* Update the contrib/realtime schemas to allow the nat column to handle the
  different nat setting combinations we have

This patch received a review and "Ship It!" on the issue itself.

(closes issue ASTERISK-20904)
Reported by: JoshE
Tested by: JoshE, Michael L. Young
Patches:
  asterisk-20904-nat-auto-and-rt-peersv2.diff Michael L. Young (license 5026)
........

Merged revisions 382322 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@382323 65c4cc65-6c06-0410-ace0-fbb531ad65f3

CHANGES

@@ -398,6 +398,9 @@ chan_sip
    detects that an incoming SIP request crossed a NAT after being sent by
    the remote endpoint.
 
+ * The default global nat setting in sip.conf has been changed from force_rport
+   to auto_force_rport.
+
  * NAT settings are now a combinable list of options. The equivalent of the
    deprecated nat=yes is nat=force_rport,comedia. nat=no behaves as before.