Log Message:
cmake: use the right ps command to get child process info on NetBSD

CMake uses ps to pid get information on childs of a process
to recursivly kill the childs. Not having KWSYSPE_PS_COMMAND
set means that CMake won't try to kill the child processes.
Not checking for the __NetBSD__ define also causes NetBSD
Sparc to use the ps command for Solaris leading to run-time
breaks, see PR pkg/57272.
This change was suggested by Valry Ushakov on PR pkg/57272.