bug#7877: sleep 5 -4

[jidanni]

$ sleep 5 -4
sleep: invalid option -- '4'
$ sleep -- 5 -4
sleep: invalid time interval `-4'

No fair prejudicing negative numbers.

At least document it.
'However, GNU `sleep' accepts arbitrary floating point numbers (using a
period before any fractional digits).' is what it says on Debian.