The host plugin kept on adding entries instead of overwriting. Split the
inline cache off from the /etc/hosts file cache and clear /etc/hosts
file cache and re-parsing.

A bunch of other cleanup as well. Use functions defined in the plugin
package, don't re-parse strings if you don't have to and use To4() to
check the family for IP addresses. Fix all test cases a carried entries
are always fqdn-ed. Various smaller cleanup in unnessacry constants.

Fixes: #3014
Signed-off-by: Miek Gieben <miek@miek.nl>