IPv4/6 Improvements.

[martafolf]

Allow the per-host configuration to specify the host capability. (E.g. v4 or v6 only, or both)

Use that data to automatically parse input from user, and de-select hosts that are not capable to run the requested query.

For example MTR to v6 address from a v4 only host will result in no output, which can be avoided using this added check.

Potentially make this work the other way around as well, so a v6-only host will not display failed MTR to a v4 IP. (Considering the uprise of v6 only VPS from various providers, this may come in handy as more providers do it.)