Don't require read lock for 'opam config env', 'opam config exec'

Since it's more likely to cause than to avoid problems, let's be pragmatic.
Closes #1830
Better behaviour on 'pin edit' with a bad file

and remove that debug message, oops
Add dev-repo to opam template

Closes #1883
Allow http-pinning
Merge pull request #1885 from OCamlPro/more-pins

Some fixes on pin edit and file locks, allow http pinning