AST-2013-003: Prevent username disclosure in SIP channel driver

When authenticating a SIP request with alwaysauthreject enabled, allowguest
disabled, and autocreatepeer disabled, Asterisk discloses whether a user
exists for INVITE, SUBSCRIBE, and REGISTER transactions in multiple ways. The
information is disclosed when:
 * A "407 Proxy Authentication Required" response is sent instead of a
   "401 Unauthorized" response
 * The presence or absence of additional tags occurs at the end of "403
   Forbidden" (such as "(Bad Auth)")
 * A "401 Unauthorized" response is sent instead of "403 Forbidden" response
   after a retransmission
 * Retransmission are sent when a matching peer did not exist, but not when a
   matching peer did exist.

This patch resolves these various vectors by ensuring that the responses sent
in all scenarios is the same, regardless of the presence of a matching peer.

This issue was reported by Walter Doekes, OSSO B.V. A substantial portion of
the testing and the solution to this problem was done by Walter as well - a
huge thanks to his tireless efforts in finding all the ways in which this
setting didn't work, providing automated tests, and working with Kinsey on
getting this fixed.

(closes issue ASTERISK-21013)
Reported by: wdoekes
Tested by: wdoekes, kmoore
patches:
  AST-2013-003-1.8 uploaded by kmoore, wdoekes (License 6273, 5674)
  AST-2013-003-10 uploaded by kmoore, wdoekes (License 6273, 5674)
  AST-2013-003-11 uploaded by kmoore, wdoekes (License 6273, 5674)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@384003 f38db490-d61c-443f-a65b-d21fe96a405b