LibC: add wait and waitpid

Note that wait() doesn't work since only waiting for specified
pid is supported. wait() will just return -1 and set errno to
ECHILD.

libc/include/sys/syscall.h

@@ -21,6 +21,7 @@ __BEGIN_DECLS
 #define SYS_SLEEP 14
 #define SYS_EXEC 15
 #define SYS_REALLOC 16
+#define SYS_WAIT 17
 
 __END_DECLS
 

libc/include/sys/wait.h

@@ -20,11 +20,12 @@ __BEGIN_DECLS
 #define WNOWAIT		0x10
 #define WSTOPPED	0x20
 
+#define WEXITSTATUS(status)	(status & 0xFF)
+#define WIFEXITED(status)	(WEXITSTATUS(status) != 0)
+
 // FIXME
 #if 0
-#define WEXITSTATUS
 #define WIFCONTINUED
-#define WIFEXITED
 #define WIFSIGNALED
 #define WIFSTOPPED
 #define WSTOPSIG