Add a property to the CHANNEL method to retrieve the auth method,
which can be used to retrieve the specific auth method actually
negotiated for a call (e.g. RSA, MD5, etc.).
Also clean up some of the documentation for the secure properties
to clarify how these relate to call encryption.
Resolves: #1878
UserNote: CHANNEL(auth_method) can now be used to retrieve the
auth method negotiated for a call on IAX2 channels.
60219d49c9