Format certificates prettier in the NSM

(nsm-format-certificate): Format certificate prettier.  Format
suggested by Toke Høiland-Jørgensen <toke@toke.dk>.
Tweak meaning of self-signed certificates in gnutls

* gnutls.c (Fgnutls_peer_status): SIGNER_NOT_FOUND probably means
:self-signed more than SIGNER_NOT_CA does.
(nsm-check-tls-connection): Make `high' do certificate pinning

(nsm-check-tls-connection): Make `high' do certificate pinning,
and `paranoid' query for every new host.
(nsm-level): New function.
Revert commit 75ccb64a946a6668288aee50508888be392a0e7f

Exporting running_async_code does not help the NSM user interface