Support suppression of dependency checking

- Add a syntax (+packagename) to force inclusion as a dependency,
  even if it's not present in the APT cache.  This is probably a bad
  idea, but we need it for, e.g., things in our repository, since
  chdist can't support multiple sources.  To minimize potential
  damage, this syntax is only allowed in distro-specific list files.
- Note in the changelog that the athinfo defs file was removed in 1.2
Force inclusion of openfoam packages

- Since dependency-checking can't be used for packages not in upstream
  repositories, force the packages (originally added in b901af92) to
  be included
Provide a libctl3 symlink for the mpb package

Per alexp, the mpb package is looking in /usr/share/libctl3, but
the "libctl3" package installs things in /usr/share/libctl.  This
creates the necessary symlink.  It should be removed once the upstream
package is fixed