Hosts formatting

This pull request is nearly a complete rewrite of the hosts plugin to support the metadata formatting proposal described in doc/METADATA.ini and discussed in #118. Additionally it introduces a first proposal for a library (keymetaformatting) that encapsulates useful functions to create the comment meta keys.

 Although I could have spent even more time on this topic, I think it is time for a first review.
Yes, I think so. See network.c:
`hints.ai_family = AF_UNSPEC;     /* Allow IPv4 or IPv6 */`

The glob should be `/*/*` because of the new hierarchy layer (I fixed it). I have absolutely no clue what the validation/regex is meant to do, however. Most likely it isn't working anyway because of the same hierarchy problem.