[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.

Author: iabyn

doio.c

@@ -1789,12 +1789,15 @@ nothing in the core.
 	    if (!(SvNIOK(*mark) || looks_like_number(*mark)))
 		Perl_croak(aTHX_ "Can't kill a non-numeric process ID");
 	    proc = SvIV_nomg(*mark);
-	    if (killgp)
-	    {
-                proc = -proc;
-	    }
 	    APPLY_TAINT_PROPER();
-	    if (PerlProc_kill(proc, val))
+#ifdef HAS_KILLPG
+            /* use killpg in preference, as the killpg() wrapper for Win32
+             * understands process groups, but the kill() wrapper doesn't */
+            if (killgp ? PerlProc_killpg(proc, val)
+                       : PerlProc_kill(proc, val))
+#else
+            if (PerlProc_kill(killgp ? -proc: proc, val))
+#endif
 		tot--;
 	}
 	PERL_ASYNC_CHECK();