Do not always set stdin to FNONBLOCK when PICO_PLATFORM=host

Because the UART simulation was setting FNONBLOCK flag on stdin, all
stdio functions like scanf() or fgets() were non blocking.
The workaround is to use a non_blocking_getchar() function to do the UART
simulation, which will temporarily set stdin to non blocking.
This will make stdio functions blocking like they normally do.

Author: czarnota

src/host/hardware_uart/uart.c

@@ -48,12 +48,24 @@ void _inittty(void) {
     //_tty.c_oflag &= ~ONLCR;
     tcsetattr(STDIN_FILENO, TCSANOW, &_tty);
 
-    fcntl(STDIN_FILENO, F_SETFL, FNONBLOCK);
     atexit(_resettty);
 }
 
+static int non_blocking_getchar(void) {
+    fcntl(STDIN_FILENO, F_SETFL, FNONBLOCK);
+
+    int c = getchar();
+
+    int old = fcntl(STDIN_FILENO, F_GETFL);
+
+    fcntl(STDIN_FILENO, F_SETFL, old & ~FNONBLOCK);
+
+    return c;
+}
+
 #else
 void _inittty() {}
+static int non_blocking_getchar(void) { return getchar(); }
 #endif
 
 typedef struct {
@@ -67,7 +79,7 @@ static int _nextchar = EOF;
 
 static bool _peekchar() {
     if (_nextchar == EOF) {
-        _nextchar = getchar();
+        _nextchar = non_blocking_getchar();
     }
     return _nextchar != EOF;
 }