GnuTLS C support for nsm.el

[Ted Zlatanov]

Since 3.0.13, GnuTLS has gnutls_verify_stored_pubkey() and
gnutls_store_pubkey() according to
https://www.gnutls.org/manual/html_node/Certificate-verification.html
which can replace a good chunk of the code in nsm.el and move it down
the stack.

The UI is similar to the typical SSH hostkey acceptance. By default
these use ~/.gnutls/known_hosts but Emacs could override the backend
storage function to use the ~/.emacs.d/network-security.data file.

I am neutral about this, but wanted to bring it up for discussion. Does
anyone think this GnuTLS facility is valuable enough to start using it,
or should we keep nsm.el the way it is, Emacs Lisp only? Lars?

Thanks
Ted