Rename renamed nsm.el variables and functions back

Rename renamed nsm.el variables and functions back

* doc/emacs/misc.texi (Network Security): Start working on
updating the NSM bits, but it's unclear how much of the new stuff
to document.

* lisp/net/nsm.el: Rename all nsm-tls-check-* functions to
nsm-protocol-check--* to bring them back into line with the
documentation.
(network-security-protocol-checks): Renamed back again from
`nsm-tls-checks', as this variable is documented and can't just go
away.