Add .skip script-arg for http-server-header

Because http-server-header grabs the server header and sets the service
version hardmatched, Nmap won't print a service fingerprint. This means
people might not submit as many fingerprints in the future, but we would
rather they did (speeds up scans by short-circuiting version probes).

Now http-server-header will print a messages suggesting the use of
--script-args http-server-header.skip for the purposes of submitting a
fingerprint. If this script-arg is set, the script will not run,
preserving Nmap's previous behavior.