Replace ukbhit by kbd_enter_pressed, not requiring tcgetattr:

Note that it behaves differently now
* it looks for Enter key only, not any key
* it "eats" the input, no need for (void)getchar() after it
* it works the same no matter the value of ICANON
* the mingw version has been adapted to act the similarly

This should fix its usage on Android where tcgetattr always returns -1

client/util.h

@@ -129,7 +129,7 @@
 
 uint8_t g_debugMode;
 
-int ukbhit(void);
+int kbd_enter_pressed(void);
 void AddLogLine(const char *fn, const char *data, const char *c);
 void AddLogHex(const char *fn, const char *extData, const uint8_t *data, const size_t len);
 void AddLogUint64(const char *fn, const char *data, const uint64_t value);