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.
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