check_dig: patch to make dig honor -t option

Just turning attached patch of github issue #774 into a push request
Previous check_dig patch requires MATHLIBS and can cause issues if the
total timeout after division (with ceil) is larger than the timeout.

floor() is a better option, but that can be handled with INTs by just
dropping the remainer. This removes the reliance on MATHLIBS and
also provides a test for computed timeout values less than 1.

Also some usage and help additions.