Kernel: Add support for the WSTOPPED flag to the waitpid() syscall.

This makes waitpid() return when a child process is stopped via a signal.
Use this in Shell to catch stopped children and return control to the
command line. :^)

Fixes #298.

Kernel/UnixTypes.h

@@ -3,6 +3,10 @@
 #include <AK/Types.h>
 
 #define WNOHANG 1
+#define WUNTRACED 2
+#define WSTOPPED WUNTRACED
+#define WEXITED 4
+#define WCONTINUED 8
 
 #define R_OK 4
 #define W_OK 2