The apropos software is useful to get a list of manpages matching a
description or to get a list of all manpages. The latter feature is
used by Emacs to get manpage completion (`M-x man`).
To have apropos working, a database of all available manpages must be
built with mandb. This is what this commits does.
A similar change was done for NixOS:
edc6a76cc0
