KWSys Process: Add OpenBSD process tree kill support

Patch from Chuck Atkins <chuck.atkins@kitware.com>

ProcessUNIX.c

@@ -2374,7 +2374,7 @@ static pid_t kwsysProcessFork(kwsysProcess* cp,
    corresponding parsing format string.  The parsing format should
    have two integers to store: the pid and then the ppid.  */
 #if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) \
-   || defined(__FreeBSD_kernel__)
+   || defined(__FreeBSD_kernel__) || defined(__OpenBSD__)
 # define KWSYSPE_PS_COMMAND "ps axo pid,ppid"
 # define KWSYSPE_PS_FORMAT  "%d %d\n"
 #elif defined(__hpux) || defined(__sun__) || defined(__sgi) || defined(_AIX) \