Improve chan_sip's ability to determine what methods should and should not be used in a dialog.

The previous effort here was to store what a peer is capable of receiving by parsing REGISTER
requests from the peer and keeping that information for as long as the registration was active.
The problem with this is that there are a great number of SIP devices which give no indication
of the methods allowed in their REGISTER requests, and it is unreasonable to try to guess what
the device may or may not support. In addition, some SIP devices have been found to claim support
for a specific method, but their handling the method is less than ideal, or they are actually
lying.

With this patch, we now determine what methods a device supports  by parsing the Allow header we
receive from them, and we do this with each new dialog. In addition, a configuration option has
been added so that an administrator can essentially blacklist certain methods from being used
with certain peers if the admin knows that support for a specific method is dodgy or nonexistent.

ABE-1822



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@209673 65c4cc65-6c06-0410-ace0-fbb531ad65f3

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