sipmsg: allow NULL authentication name

As no authentication scheme was specified sipmsg_find_auth_header()
can only fail.
core: add SIPE_AUTHENTICATION_TYPE_AUTOMATIC
purple: add automatic authentication scheme option
transport: add automatic authentication scheme

When the user has selected automatic authentication scheme then accept
in priority order TLS-DSK, Kerberos and NTLM. Restart the automatic
selection on every server connect and full reauthentication.

NOTE: this does not handle authentication failure fallback yet.
telepathy: add automatic authentication scheme option
transport: add automatic authentication fallback

When the user has selected automatic authentication scheme and the
server rejects our authentication for the current scheme then try the
next scheme in priority order.
Fix #245: "Unable to resolve DNS SRV record" error when joining conference

Not all Lync conference URL's have a 3-part domain name. Replace the
hard-coded solution in parse_lync_join_url() with a more generic one
that only drops the first part of the domain name.

Also add debug logging to sipe_core_conf_create(),
transport: add missing flagging for Kerberos

We shouldn't automatically try Kerberos if there is no support for it
compiled in.
transport: add automatic authentication fallback (II)

When the user has selected automatic authentication scheme and the
current scheme fails to initialize or can't continue then don't
generate an Authorization header for the REGISTER attempt. The server
will reject this with a 401 response which in turn triggers SIPE to try
the next scheme in priority order.
ews: extract settings also from type EXPR

It seems that Microsoft has changed the EWS autodiscover response for
Office 365. In my test account the type EXCH now only contains the OAB
URL, whereas the type EXPR now contains the same OAB URL and the other
three URLs. So SIPE ends up with no EWS URL and therefore can't extract
the UCS buddy list :-(

Update the response parser to loop over all types and use for each URL
the first found value from EXCH or EXPR.
miranda: add automatic authentication scheme option
adium: add automatic authentication scheme option
Fix #248: Remove libpurple SSL configure check

On some installations the stripped-down libpurple initialization is not
enough and causes the SSL configure check to fail. A better solution
would be to call purple_core_init(), but that fails on other platforms.

The only option is to remove the configure check completely. Not a big
loss anyway...
sdpmsg: don't send farsight-send-profile codec property

Another instance of a Lync client being picky about SDP parameters.
Some versions of Lync AVMCU (conferencing server) reject messages that
contain the send profile property added by Farstream, likely because
they treat spaces or exclamation marks as illegal characters.