Revert Windows test watchdog() to kill('KILL', ...)

Revert Windows test watchdog() to kill('KILL', ...)

There are suspicions that the process tree kill('-KILL', ...) might be
nuking too much. It was only done to kill the cmd.exe+perl.exe tree that
was unexpectedly launched by system(1, $cmd), but by switching to
system({$perl} 1, $perl, '-e', $prog) we can avoid the intermediate cmd.exe
and thus revert to normal process kill('KILL', ...) instead to kill the
perl.exe that is now launched directly.

See http://www.nntp.perl.org/group/perl.perl5.porters/2015/05/msg227859.html