Add the BSD version of GetCommandForPid()

- tested on Net, Free, Open and DragonFly

- fix up the linux version (terminate argv with a null ptr)
- add error handling to file ops in the linux version
- add a stub version for platforms not supported

src/osdep/stub.c

@@ -0,0 +1,11 @@
+
+#include <WINGs/WUtil.h>
+#include "../wconfig.h"
+
+Bool GetCommandForPid(int pid, char ***argv, int *argc)
+{
+	*argv = NULL;
+	*argc = 0;
+
+	return False;
+}