Gtalk enhancements and general code cleanup.

This patch includes several chan_gtalk enhancements.
Two new gtalk.conf options have been added, externip
and stunadd.  Setting externip allows us to
manually specify what the external IP address is
outside of a NAT environment.  Setting the stunaddr
option to a valid stun server allows for that external
ip to be retrieved via a STUN server automatically.  This
external IP is then advertised during call setup as
a possible candidate.

I have also attempted to clean up chan_gtalk's code
so it meets our coding guidelines. During this cleanup
I noticed several things that need to be done in the
code and made a TODO section at the top of the file.




git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@291192 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/stun.h

@@ -32,6 +32,8 @@
 extern "C" {
 #endif
 
+static const int STANDARD_STUN_PORT = 3478;
+
 enum ast_stun_result {
 	AST_STUN_IGNORE = 0,
 	AST_STUN_ACCEPT,