hints.add_hosts(path): a new property

So far it wasn't possible to load multiple files (!).  Real use case:
https://forum.turris.cz/t/how-to-configure-local-address-dns-resoultion-on-omnia/1000/14
3 jobs for <span class="ref-name">hints-fixes</span> in 1 minute and 47 seconds (queued for 28 seconds)
Status Job ID Name Coverage
  Build
passed #1870
docker linux amd64
build:linux:amd64

00:00:29

 
  Test
passed #1872
docker linux amd64
deckard:linux:amd64

00:00:55

passed #1871
docker linux amd64
test:linux:amd64

00:00:22