Make lukko flag automatic and off by default

The lukko package (https://hackage.haskell.org/package/lukko)
was meant to be a temporary workaround, and relevant improvements
have been long accessible from base:GHC.IO.Handle.Lock.

As suggested at haskellari/lukko#39 (comment),
let's switch the default value of the lukko flag of cabal-install
from True to False. Additionally, to be on the safe side, we make it
an automatic one, so that if things go terribly wrong a Hackage revision
could sort it back.

The same change has been implemented for the hackage-security package
sometime ago in haskell/hackage-security#322,
and there seems to be no complaints.

Author: Bodigrim

cabal-install/cabal-install.cabal

@@ -36,8 +36,8 @@ Flag native-dns
 
 Flag lukko
   description:  Use @lukko@ for file-locking
-  default:      True
-  manual:       True
+  default:      False
+  manual:       False
 
 flag git-rev
   description: include Git revision hash in version