Use a simple list of symbols in GnuTLS peer verification.

* gnutls.c (Fgnutls_peer_status_warning_describe): Add function to describe a
peer verification warning symbol.
(Fgnutls_peer_status): Use it.
(Fgnutls_boot): Use it.
net/nsm.el: Use `gnutls-peer-status-warning-describe'.

* net/nsm.el (nsm-check-tls-connection, nsm-save-host)
(nsm-warnings-ok-p): Use `gnutls-peer-status-warning-describe'.
* gnutls.c (Fgnutls_boot): Minor logging fixes.
Make gnutls.c compile on older gnutls installations

* gnutls.c (gnutls_certificate_details): Don't include certain
certificate details on older gnutls versions (bug#19168).
(syms_of_gnutls): Export gnutls_peer_status_warning_describe.
* net/nsm.el (nsm-format-certificate): Don't bug out on missing elements
(gnutls_certificate_details): Reversed check in last checkin.
Fix previous version of nsm-warnings-ok-p

* net/nsm.el (nsm-format-certificate): Don't bug out on missing
elements.
(nsm-warnings-ok-p): The new version of this function always
returned nil when everything was OK.
* misc.texi (Network Security): Use "untrustworthy" instead of "unsafe"
Add README.md.