idr: Remove duplicated code

* idr_get_new() and idr_get_new_above() mostly do the same thing

* Have idr_get_new() call idr_get_new_above() and only keep a single
  implementation
bsd.man.mk: add temporary ``mandiff'' target

Show differences in ASCII output between groff(1) and mandoc(1).  This
will help generate uptream bug reports.  Also useful for spotting local
configuration quirks of both programs.
mdocml: sync a few upstream commits

Most of our local changes have found its way to upstream.
Let's return the favour and bring in fixes that we have
requested.  Hunk count for mandiff output reduced from
3321 to 3311 (not much, but it's a start).

     # make mandiff | grep "^@@" | wc -l