Return nil, not false, from firewall-bypass.nse
Handle ICMP Time Exceeded messages in scan engine

DiabloHorn on #nmap noticed that Nmap was ignoring ICMP Time Exceeded
messages while trying to map firewall rules using --ttl.
get_pcap_result() was handling ICMP type 3 (Destination Unreachable),
but not type 11 (Time Exceeded). Now ports that elicit this response
will be marked filtered (to be consistent with existing Connect scan
behavior) and will report time-exceeded from (IP) for the reason.

This was not a common issue, since host discovery already accounted for
it. Port scans would only be affected when skipping host discovery.
Changelog entry for time-exceeded port reason