util: Fix the ispathsub corner case

ispathsub("/foo", "/") reports false. This is a corner case,
as 2nd argument is not expected to end with /. Fix this and
add comment about ispathsub() arguments assumptions.

Reported-by: Andrey Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>