Emacs core TLS support

[Ted Zlatanov]

Nearly ready.  Since the last patch we have:

- full initialization and handshake (no memory issues, etc.)

- everything happens in gnutls-boot, including global initialization;
  all the parameters are either x509 or anon

- use of gnutls_initstage in the process to mark progress of
  initialization and whether the process is done initializing and
  handshaking

- no SRP anywhere, just anon and x509 (I'll add SRP if we need it and
  when the other two are working)

Now I get GNUTLS_E_INSUFFICIENT_CREDENTIALS when I open a x509
connection to an IMAP TLS server so I think there's still work to do.
The trust file seems to be wrong (see lisp/net/gnutls.el, I tried both
"/etc/ssl/certs/ca-certificates.crt" and "/etc/ssl/certs/ca.pem").

The GnuTLS examples don't seem to cover the standard situation of
talking to a web server over SSL and possibly accepting an insecure
connection if the server credentials are bad.  I must have missed
something.  Could the GnuTLS developers look at my patch and help me
out?

Thanks
Ted