[perl #121230] fix kill -SIG on win32

v5.17.1-137-gc2fd40c tidied up perl's kill() implementation, making
-SIGNAME be handled correctly, It also eliminated the use of the killpg()
system/library call, which these days is usually just a thin wrapper over
kill(). By doing this, it assumed that the following are functionally
equivalent:

 killpg(pgrp, sig)
 kill(-pgrp, sig).

Unfortunately this broke Window's use of the (perl-level)

    kill negative_value, pid, ...

since under Windows, the killpg()/kill() identity doesn't hold
(win32_kill() uses negative ids for fake-PIds, not process groups).

Fix this by restoring the use of killpg() where available.