Make manual generation not depend on the user's configuration

Previously changing the value of certain options in configuration.nix
would cause the manual to be regenerated, because some option defaults
depend on the configuration.  This is undesirable because rebuilding
the manual is kind of slow.  So now the manual reflects an empty
configuration.

The downside is that this requires another call to fixMergeModules,
which adds about 14% to evaluation time.  Probably worth it.