Replace ps for pgrep to list evobackup child procs

The command being used to identify evobackup child processes isn't
supported on OpenBSD. The pgrep command is supported on both Linux and
OpenBSD.

zzz_evobackup

@@ -114,7 +114,7 @@ pick_server() {
 if [ -e "${PIDFILE}" ]; then
     pid=$(cat "${PIDFILE}")
     # Killing the childs of evobackup.
-    for ppid in $(ps h --ppid "${pid}" -o pid | tr -s '\n' ' '); do
+    for ppid in $(pgrep -P "${pid}"); do
         kill -9 "${ppid}";
     done
     # Then kill the main PID.