Make kill() fatal for non-numeric pids

Make kill() fatal for non-numeric pids

As the debate over the best way to deal with floating point
pids stalled, this is just for non-numeric, which at least
squashes the bug even if it's not the Platonic ideal for
everyone.

It also doesn't address overloaded objects that might not have
IV, NV or PV appropriately set, but the approach mirrors what is
done elsewhere in doio.c so I recommend applying this patch now and
fixing the problem of overloaded objects at some other time when
it can be done more globally, either through an improvement or
replacement of looks_like_number

Also updated POD for kill when process is 0 or negative and
fixed Test-Harness tests that used kill with a string pid.
(Test-Harness test fix also submitted upstream)