tweaks: use a symbol instead of zero to refer to standard input

src/files.c

@@ -827,7 +827,7 @@ void read_file(FILE *f, int fd, const char *filename, bool undoable)
 
 	/* When reading from stdin, restore the terminal and reenter curses mode. */
 	if (isendwin()) {
-		if (!isatty(0))
+		if (!isatty(STANDARD_INPUT))
 			reconnect_and_store_state();
 		terminal_init();
 		doupdate();

src/nano.c

@@ -1078,7 +1078,7 @@ void reconnect_and_store_state(void)
 	if (thetty < 1)
 		die(_("Could not reconnect stdin to keyboard\n"));
 
-	dup2(thetty, 0);
+	dup2(thetty, STANDARD_INPUT);
 	close(thetty);
 
 	/* If input was not cut short, store the current state of the terminal. */
@@ -1096,7 +1096,7 @@ bool scoop_stdin(void)
 	tcsetattr(0, TCSANOW, &oldterm);
 
 	/* When input comes from a terminal, show a helpful message. */
-	if (isatty(0))
+	if (isatty(STANDARD_INPUT))
 		fprintf(stderr, _("Reading data from keyboard; "
 							"type ^D or ^D^D to finish.\n"));
 

src/nano.h

@@ -146,6 +146,8 @@
 #define BAD_COLOR -2
 #endif
 
+#define STANDARD_INPUT  0
+
 /* Enumeration types. */
 typedef enum {
 	NIX_FILE, DOS_FILE, MAC_FILE