Move unconditional `man_MANS` -> `dist_man_MANS`

Move unconditional `man_MANS` -> `dist_man_MANS`

* Avoids the repetition in `EXTRA_DIST`, and is the recommended
  Automake pattern.
* Some `man_MANS` will have to remain, since Automake has a bug
  where `dist_man_MANS` in Automake branches do not get included
  in the tarball:
    https://lists.gnu.org/archive/html/bug-automake/2008-06/msg00019.html