commit.rc: Add helper for math operation using bc

commit.rc: Add helper for math operation using bc

Sometimes using bash $(()) math might not be enough due to some
limitation (big numbers), so add helper using 'bc' program. For
now the results are only in perfect numbers (as in bash) since this is
all I need for now.

This commit also adds _require_math() helper which should be called by
every test which uses _math() since it requires "bc" to be installed on
the system.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>