Sync with OpenBSD.

bc.y: Rev 1.50
- write parse errors to stderr, prompted by Martijn Dekker
- we're only interactive if stdout en stderr are a tty as well as stdin

PR:		234430
Obtained from:	OpenBSD
MFC after:	1 week

usr.bin/bc/bc.y

@@ -996,7 +996,7 @@ yyerror(const char *s)
 			putchar('\\');
 		putchar(*p);
 	}
-	fputs("]pc\n", stdout);
+	fputs("]ec\n", stdout);
 	free(str);
 }
 
@@ -1173,7 +1173,8 @@ main(int argc, char *argv[])
 	argc -= optind;
 	argv += optind;
 
-	interactive = isatty(STDIN_FILENO);
+	interactive = isatty(STDIN_FILENO) && isatty(STDOUT_FILENO) &&
+	    isatty(STDERR_FILENO);
 	for (i = 0; i < argc; i++)
 		sargv[sargc++] = argv[i];