Fix many GCC -Wimplicit-fallthrough warnings (#243)

This commit adds '/* FALLTHROUGH */' comments to fix many
GCC warnings when compiling with -Wimplicit-fallthrough.
Additionally, the existing fallthrough comments have been
changed for consistency.

src/cmd/builtin/pty.c

@@ -1037,6 +1037,7 @@ b_pty(int argc, char** argv, Shbltin_t* context)
 			continue;
 		case 'l':
 			log = opt_info.arg;
+			/* FALLTHROUGH */
 		case 'm':
 			messages = opt_info.arg;
 			continue;