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.
you wrote:
> Yes, I think so. See network.c:
> `hints.ai_family = AF_UNSPEC;     /* Allow IPv4 or IPv6 */`

I was rather interested in a test if glob+network+hosts really all work 
together with 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.

Thats kind of a type checker in Elektra that allows to check values according 
regular expression.

network is also a type checker, but superior for ip addresses because it 
exactly tests if the interface would accept it.