Allow multiple portable instances of uTox to run concurrently

[robinlinden]

https://github.com/uTox/uTox/blob/develop/src/windows/main.c#L811
static bool win_init_mutex(HANDLE *mutex, HINSTANCE hInstance, PSTR cmd) needs to be updated. Wouldn't be much work. Check if we're in portable mode or something along those lines and use pwd for the mutex if we are.