This is in order to better support derivative engines.
Rework the manpages to be parameterized with @PROGRAM_PREFIX@ and
@PACKAGE_NAME@. Alter the `docgen` generator to substitute those
parameters. Supply those parameters via the autofoo Makefile.
Adjust the fixed path chocolate-server.6 to be generated from
server.template, similar to doom.template -> chocolate-doom.6.
Further work is required, not least renaming chocolate-setup.6,
but several further parameterizations.
