unix,windows: Use STDIN_FILENO, STDOUT_FILENO macros where appropriate.

This replaces 0 and 1 with STDIN_FILENO and STDOUT_FILENO to make the
intention of the code clearer.

ports/minimal/uart_core.c

@@ -17,7 +17,7 @@ typedef struct {
 int mp_hal_stdin_rx_chr(void) {
     unsigned char c = 0;
     #if MICROPY_MIN_USE_STDOUT
-    int r = read(0, &c, 1);
+    int r = read(STDIN_FILENO, &c, 1);
     (void)r;
     #elif MICROPY_MIN_USE_STM32_MCU
     // wait for RXNE
@@ -31,7 +31,7 @@ int mp_hal_stdin_rx_chr(void) {
 // Send string of given length
 void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
     #if MICROPY_MIN_USE_STDOUT
-    int r = write(1, str, len);
+    int r = write(STDOUT_FILENO, str, len);
     (void)r;
     #elif MICROPY_MIN_USE_STM32_MCU
     while (len--) {

ports/unix/unix_mphal.c

@@ -167,7 +167,7 @@ main_term:;
 
     MP_THREAD_GIL_EXIT();
     unsigned char c;
-    int ret = read(0, &c, 1);
+    int ret = read(STDIN_FILENO, &c, 1);
     MP_THREAD_GIL_ENTER();
     if (ret == 0) {
         c = 4; // EOF, ctrl-D
@@ -179,7 +179,7 @@ main_term:;
 
 void mp_hal_stdout_tx_strn(const char *str, size_t len) {
     MP_THREAD_GIL_EXIT();
-    int ret = write(1, str, len);
+    int ret = write(STDOUT_FILENO, str, len);
     MP_THREAD_GIL_ENTER();
     mp_uos_dupterm_tx_strn(str, len);
     (void)ret; // to suppress compiler warning

ports/windows/windows_mphal.c

@@ -222,7 +222,7 @@ int mp_hal_stdin_rx_chr(void) {
 
 void mp_hal_stdout_tx_strn(const char *str, size_t len) {
     MP_THREAD_GIL_EXIT();
-    write(1, str, len);
+    write(STDOUT_FILENO, str, len);
     MP_THREAD_GIL_ENTER();
 }