Avoid bitshifts in macros for static error codes

Exit codes of command not found and exec related errors are static
values and should not require any macro magic for calculation.

Related: #485

Author: siteshwar

src/lib/libast/include/ast.h

@@ -51,10 +51,10 @@
 
 #define EXIT_BITS 8 /* # exit status bits	*/
 
-#define EXIT_USAGE 2                               /* usage exit code	*/
-#define EXIT_QUIT ((1 << (EXIT_BITS)) - 1)         /* parent should quit	*/
-#define EXIT_NOTFOUND ((1 << (EXIT_BITS - 1)) - 1) /* command not found	*/
-#define EXIT_NOEXEC ((1 << (EXIT_BITS - 1)) - 2)   /* other exec error	*/
+#define EXIT_USAGE 2            /* usage exit code	*/
+#define EXIT_QUIT 255           /* parent should quit	*/
+#define EXIT_NOTFOUND 127       /* command not found	*/
+#define EXIT_NOEXEC 126         /* other exec error	*/
 
 #define EXIT_CODE(x) ((x) & ((1 << EXIT_BITS) - 1))
 #define EXIT_CORE(x) (EXIT_CODE(x) | (1 << EXIT_BITS) | (1 << (EXIT_BITS - 1)))