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'.