Add gnutls_enable_pkcs11 option.

Add gnutls_enable_pkcs11 option.

GnuTLS 2.12.0 adds PKCS11 support using p11-kit and by default will
autoload modules, which interoperates badly with GNOME keyring
integration, configured via paths in environment variables, and Exim
invoked by the user (eg, mailq) will then try to load the modules, fail
and spew warnings from the module for a library loaded by a library.

http://www.gnu.org/software/gnutls/manual/gnutls.html#Smart-cards-and-HSMs
documents that to prevent this, explicitly init PKCS11 before calling
gnutls_global_init().  So we do so, unless the admin sets the new
option.

Reported by Andreas Metzler, who confirmed that the added calls fixed
the problem for him.