Add a mirage-dns virtual package that checks that dns.mirage is available
refine conduit-mirage
fix build rule for mirage-dns
relax the cowabloga omd constraint
rename conduit-mirage to mirage-conduit for consistency with other mirage-* packages
mirage-conduit now depends on mirage-dns
Merge pull request #38 from avsm/master

Improves for mirage-* libraries