tools: checkusage can dump all command output

tools: checkusage can dump all command output

use
$ make CU_DUMP=1 checkusage

for easily reviewing and comparing all output in
  /tmp/checkusage--help
  /tmp/checkusage--version
  /tmp/checkusage--unknownopt

This was a big help when doing all my last usage cleanup.
Actually I used it to dump the original output, edited it
how I would like it, and then changed our programs to match
the wanted output.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>