tools: make checkmans.sh to find missing manuals

Finding missing manuals is loosely based on ideas in man-page-day-1.sh,
wrote by Peter Schiffer.

CC: Peter Schiffer <pschiffe@redhat.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: add mkfs.cramfs manual page

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: add fsck.cramfs manual page

Signed-off-by: Sami Kerola <kerolasa@iki.fi>