Add a 'redirect' field in repo files

At the moment, this is just a no-op
more informative curl error messages
closes #905
Merge pull request #910 from avsm/cold-bootstrap

Support `make cold` to build OPAM without an OCaml compiler being available
Merge pull request #909 from AltGr/better-curl-messages

more informative curl error messages closes #905
sync CHANGES
Merge pull request #911 from avsm/master

sync CHANGES
Long lines
Minor refactoring
Move the filter functions in their own file
Implement the redirect logic
Fixed bug where changes to the package could not be seen after update
Closes #894
quick-fix to opamRT

partial revert of a27f427a1efa887e849a541e969fb2caabb1f41d
Merge pull request #913 from AltGr/fix-unseen-update

Fixed bug where changes to the package could not be seen after update (#894)
Merge pull request #914 from AltGr/opamRT-quick-fix

quick-fix to opamRT
Redirection works.

Close #892
Merge pull request #912 from samoht/892

First draft for the 'redirect' feature
Remove redundant and misleading error messages with bad package archives
Closes #920
Change yellow color that is sometimes difficult to read
Closes #918
Merge pull request #926 from AltGr/fix-bad-checksum-internal-error

Remove redundant and misleading error messages with bad package archives Closes #920
Merge pull request #927 from AltGr/opam-list-magenta

Change yellow color that is sometimes difficult to read