Nmap crashes with "Error compiling our pcap filter: icmp6 not supported"

[dmiller-nmap]

Older libcpaps had IPv6 disabled by default. When this is the case, compiling a pcap filter with "ip6" or "icmp6" in it will fail, and libnetutil treats this as a fatal error. It would be great if we could check for this at compile time (or maybe even runtime) and avoid the crash, but then we have to determine how to handle it when we can't set an appropriate pcap filter.