When using --with-config-file, all man-db programs completely ignore the
systemwide configuration in /etc/man_db.conf: it means on NixOS there is
no way to change the configuration without rebuilding man-db, which in
turn causes a mass-rebuild.

To solve this problem this commit removes the NixOS-specific
configuration in man-db, which wasn't the appropriate place to begin
with: the package is expected to work on non-NixOS systems as well. Also
a small patch now ensure /etc/man_db.conf is used, if available, before
the bundled configuration.

Previously the NixOS-specific configuration for man-db was in the
package itself and /etc/man.conf was completely ignored.
This change moves it to /etc/man_db.conf, making declarative
configuration practical again.

It's now possible to generate the mandb caches for all packages
installed through NixOS `environment.systemPackages` at build-time.
The standard location for the stateful cache (/var/cache/man) is also
configured to allow users to run `mandb` manually if they wish.

Since generating the cache can be expensive the option is off by
default.