bug#9102: "timeout 0 FOO" should timeout right away

[Paul Eggert]

"sleep 0" sleeps for zero seconds, and "timeout 0 FOO"
should timeout in zero seconds as well.  Currently,
it doesn't; it times out in an infinite number of seconds.
I see why, from the internals (alarm (0) is a special
call intended to cancel alarms).  However, 'timeout' shouldn't
be exposing those internals to users; it should behave like
'sleep' does, as that's more consistent.