Avoid conflicts between groff and mdocml.

Add a 'mandoc-' prefix to roff.7 and preconv to avoid their counterparts
from groff.  This allows both packages to co-exist ready for mdocml to
be used as an nroff replacement in USE_TOOLS.
Use textproc/mdocml as the provider for nroff.

When using MKCATPAGES, there were circular dependencies in packages
which needed to generate catman pages but required groff to provide the
nroff tool, as groff has a significant number of dependencies.

mdocml is much smaller, has fewer dependencies, and is BSD licensed.
Support catman on systems which require a section.

Introduce a CATMAN_SECTION_SUFFIX variable, which if set to 'yes' will
ensure that catman pages are installed with a suffix which matches their
section instead of the default ".0".

Install bootstrap-mk-files modifications for SunOS to install catman
pages by default and to install them with the appropriate suffix.
Fix catman page installation.

  - For those which use a BSD Makefile, conditionalise the installation
    on CATMAN_SECTION_SUFFIX and ensure it is passed to the build.

  - Add USE_TOOLS+=nroff for packages which generate their catman pages.