If service scan failed to find a match, the probe responses are cached.
Now http-server-header will look through those responses before sending
a probe of its own. This should result in better detection with fewer
probes sent.
Also changed the Server header string match to case-insensitive.
1. Use "softmatched" to let Nmap print the service fingerprint for the
user to submit.
2. Run even if version detection got a good match. This allows it to be
run by-name, or to provide additional info if available. Existing match
will not be clobbered, though.
3. Use comm.lua's default timeouts. Also, no need to pass port.protocol,
since comm.tryssl will use the port table directly.
4. XML output
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.
