makedevs: fix cases where (start != 0)

The makedevs script did not always generate the requested set of device names / minor number series.
* If start != 0, then requesting (count) devices would generate only (count - start)
* If start != 0 && increment != 1, then requesting minors starting with (minor) would generate minors starting with (minor + (start * (increment - 1)))

This patch fixes the code and updates the usage text with extra examples.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>