LibC: Add WSTOPSIG macro

This macro can be used on the "status" output from sys$waitpid() to
find out which signal caused the waitee process to stop.

Fixes SerenityOS#794.

Libraries/LibC/sys/wait.h

@@ -32,6 +32,7 @@
 __BEGIN_DECLS
 
 #define WEXITSTATUS(status) (((status)&0xff00) >> 8)
+#define WSTOPSIG(status) WEXITSTATUS(status)
 #define WTERMSIG(status) ((status)&0x7f)
 #define WIFEXITED(status) (WTERMSIG(status) == 0)
 #define WIFSTOPPED(status) (((status) & 0xff) == 0x7f)