Add GnuTLS support for W32 and certificate and hostname verification in GnuTLS.

Add GnuTLS support for W32 and certificate and hostname verification in GnuTLS.

* src/gnutls.c: Renamed global_initialized to
gnutls_global_initialized.  Added internals for the
:verify-hostname-error, :verify-error, and :verify-flags
parameters of `gnutls-boot' and documented those parameters in the
docstring.  Start callback support.
(emacs_gnutls_handshake): Add Woe32 support. Retry handshake
unless a fatal error occured. Call gnutls_alert_send_appropriate
on error. Return error code.
(emacs_gnutls_write): Call emacs_gnutls_handle_error.
(emacs_gnutls_read): Likewise.
(Fgnutls_boot): Return handshake error code.
(emacs_gnutls_handle_error): New function.
(wsaerror_to_errno): Likewise.

* src/gnutls.h: Add GNUTLS_STAGE_CALLBACKS enum to denote we're in the
callbacks stage.

* src/w32.c (emacs_gnutls_pull): New function for GnuTLS on Woe32.
(emacs_gnutls_push): Likewise.

* src/w32.h (emacs_gnutls_pull): Add prototype.
(emacs_gnutls_push): Likewise.