Moving ra_serf authn handlers to serf.

As already mentioned at SubConf last week, I'm moving the code that
handles the authentication schemes in ra_serf from Subversion to Serf.

The main goal for me is to be able to add features to serf that rely
on authentication being in serf too (e.g. SSL tunneling over HTTP
proxies). It also just seems more logical to do the authentication in
the http library instead of the application, and this might help other
users of serf.

The code I want to move is parts of libsvn_ra_serf/util.c and all of
the files libsvn_ra_serf/(auth.*|auth_digest.*|auth_kerb.*|win32_auth_sspi.*).
Most of these have been written by Justin and me, but there were some
fixes, cleanups, buildscripts by other svn committers.

Both Subversion and Serf are covered by the Apache License 2.0, so I
don't think there's a problem in this area.

However, people have suggested to add a line in serf/NOTICE that says
Serf includes code developed by the Subversion Corporation.

Attached my WIP patches for serf and subversion, including the changed
NOTICE file.

Thoughts? Remarks?

Lieven

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