KWSys: Process tree kill for kFreeBSD, GNU/Hurd

kFreeBSD and Hurd have the same userland as Linux.  This change is
necessary to enable kwsysProcessKill() to kill child processes on
kFreeBSD.  The bug was detected by CTestTestTimeout test.

Patch from "Modestas Vainius <modestas@vainius.eu>".
See issue #10432.