Refactor the protocol NSM checks for flexibility

Refactor the protocol NSM checks for flexibility

* doc/emacs/misc.texi (Network Security): Mention
network-security-protocol-checks.

* lisp/net/nsm.el (network-security-protocol-checks): New variable.
(nsm-check-protocol): Refactor the checks into separate functions
for greater flexibility.
(nsm-protocol-check--diffie-hellman-prime-bits)
(nsm-protocol-check--rc4, nsm-protocol-check--ssl)
(nsm-protocol-check--signature-sha1): Refactored out of the big
function.