Merged revisions 271689 via svnmerge from

Merged revisions 271689 via svnmerge from
https://origsvn.digium.com/svn/asterisk/branches/1.4

........
  r271689 | mnicholson | 2010-06-22 07:52:27 -0500 (Tue, 22 Jun 2010) | 8 lines

  Modify chan_sip's packet generation api to automatically calculate the Content-Length.  This is done by storing packet content in a buffer until it is actually time to send the packet, at which time the size of the packet is calculated.  This change was made to ensure that the Content-Length is always correct.

  (closes issue #17326)
  Reported by: kenner
  Tested by: mnicholson, kenner

  Review: https://reviewboard.asterisk.org/r/693/
........

This change also adds an ast_str_copy_string() function (similar to ast_copy_string), that copies one ast_str into another, properly handling embedded nulls.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@271690 65c4cc65-6c06-0410-ace0-fbb531ad65f3