Add a new soeliminate(1) command

mandoc(1) does not provide an equivalent of the GNU groff's soelim(1) as an
external binary. It does provide the funcitonnality but internally.

Lots if manpages in ports uses ".so" directives to include the content of
another manpage, which works properly if the manpages are not compressed.
With compressed manpages it will fail. So we need to preprocess those manpages
with soelim(1) before compressing them.

soeliminate(1) add the minimum functionnality from soelim(1) required for that
task, in order to still be able to prepare properly those manpages in case we
ship the base system only with mandoc as a manpage renderer.

soeliminate(1) accept all the arguments from soelim(1) for compatibility but
only '-I dir' is really functionnal.

Name it soeliminate and not soelim, so groff from base or ports can still call
soelim(1) for its internal use and avoid potential incompatibilities

MFC after: 1 month