Ignore OPAMSWITCH when displaying environment varilables (eg. on 'opam config env' or 'opam switch foo').

This fixes a regression introduced by 27b2f81deb0213e12e1aa1cf833b4d991a8653f6 where you cannot switch anymore when you have OPAMSWITCH set.
Merge branch 'master' of github.com:OCamlPro/opam
Improve the error message when a constrained package is not valid.

Fix #560
A pinned package should always have the same version number.

This means:
(i) if the package is already installed, it should be the version of the installed package
(ii) otherwise abitrary pick the latest version

Previously, we didn't check (i) which was causing weird "Multiple versions installed for package ..." errors when a new version of the pinned package appears upstream.

This should help fixing bugs similar to #541 (not sure it really fixes #541, need more investigation)
On 'opam info' show when a package is pinned.

This can help debugging weird situation when we forgot that we've pinned some packages.
'opam info foo.bar' should fail if 'bar' is not a valid version for 'foo'.

This is related to #469
Return an error if opam info/list/search returns an empty result.

This is related to #469 and (vaguely) to #560
Add missing '.' in error messages
Fix comment alignment
Fix the repository the given package comes from.

Partially fix #556
Display '(pinned)' in the repository field when the given package is pinned.

This is slightly related to #556
Display information about upstream on 'opam info foo -v'

Fix #556
By default, check the certificate of downloaded dependency archives.

Use --disable-certificate-check if you want to bypass that. See #408 for instance.

Partially fix #563
Export the presence of md5 or md5sum during the configure phase
Check the md5sum of downloaded archives.

This should fix #563

Remark1: this will break the compilation of hosts which don't have md5 or md5sum installed. I'll add that in the sources.

Remark2: I've writtent md5check in ML because md5 (on OSX) and md5sum (on linux) don't have the same command-line interface.
Configure fails if md5 or md5sum are not installed
Improve OCaml script to check md5 checksums (eg. don't call md5/md5sum anymore)
Revert previous change to the configure: we don't need to check for the presence of md5/md5sum anymore.

This reverts cd5dc8faf6165184f0f1565572c6a9d8eea479eb and 09aa904d7ecacd93b46ed57cbe5cd35d4afa68d5

(and it's a better fix for #563)
Update CHANGES
Update ocp-build binary to OCamlPro/typerex@c23ec435dcea2326ace4b88e506225e61bff7591