Rework directory variables

- Directory variables as used in GNU coding standards are used by
  the bsdowl package and saved forever in variables of the form
  _BPS_PREFIX, and so on.
- These directory variables are also used in packages built by bsdowl.
- The installation procedure is made easier to configure, so that bsdowl
  interoperates better with opam.

Closes #142, #66.
Always define MANDIR

Always define the MAN product installation parameters instead
of only reading them upon inclusion of bps.man.mk.