Bug fixes and certificate and hostname verification for the Emacs GnuTLS support.

Bug fixes and certificate and hostname verification for the Emacs GnuTLS support.

* lisp/net/gnutls.el (gnutls-negotiate): Add hostname, verify-flags,
verify-error, and verify-hostname-error parameters.  Check whether
default trustfile exists before going to use it. Add missing
argument to gnutls-message-maybe call. Return return value.
Reported by Claudio Bley <claudio.bley@gmail.com>.
(open-gnutls-stream): Add usage example.

* lisp/net/network-stream.el (network-stream-open-starttls): Give host
parameter to `gnutls-negotiate'.
(gnutls-negotiate): Adjust `gnutls-negotiate' declaration.