Fixing pinned packages #1: only the pinned version is 'available'
Changed 'installed' files to contain 'pinned' for pinned packages
that way, you can change the version in the package's .opam file without breaking everything
Fixed descrs for pinned packages
(and full-descrs for all of them :p)
Fixed conversion `pinned` <-> `real_version` in opamCudf

Closes #744
Fixed unmarking of packages as roots after reinstall (closes #759)