Cleanup of flags used in struct sip_request, moving them to

Cleanup of flags used in struct sip_request, moving them to
individual variables. Apart from SIP_PKT_IGNORE which was used
a zillion times, the other two are used seldom.

On passing:
- move the arrays to the end of struct sip_request, so a (small)
  buffer overflow is less likely to overwrite the other fields;
- note that the 'ignore' argument to handle_invite_replaces() is not
  used and should be removed (will be done in a separate commit).

Nothing to backport in this change.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@76365 65c4cc65-6c06-0410-ace0-fbb531ad65f3