…back (#299)

- initial API;
- Windows backend implementation;

- fix merge conflict
- fix indentation in a few places

* netbsd hotplug stubs

* Make cygwin happy (fix copied from libusb)

…645)

…EnterCriticalSection (#689)

Fix the possible issues that happen when a register or de-register call is made from inside a callback. The way it works is the same for all platforms and is described below.

Resolves: #673 

1) The mutex is made recursive, so it can be locked by the same thread multiple times
2) `mutex_ready` kept intact, added 2 more flags, `mutex_in_use` and `cb_list_dirty`.
3) When the `mitex_in_use` flag is set, the Deregister call is no longer allowed to immediately remove any callbacks from the list. Instead, the `events` field in the callback is set to 0, which makes it "invalid", as it will no longer match any events, and the `cb_list_dirty` flag is set to 1 to indicate that the list needs to be checked for invalid events later.
4) When a callback returns a non-zero value, indicating that the callback is to be disarmed and removed from the list, it is marked in the same manner until the processing finishes (unless the callback was called directly by the Register call, in which case it's return value is ignored on purpose)
5) After all the callbacks are processed, if `cb_list_dirty` flag is set, the list of callbacks is checked for any callbacks marked for removal (`events` field set to 0), and those are only removed after all the processing is finished.
6) The Register call is allowed to register callbacks, as it causes no issues so long as the mutex it locks is recursive
7) Since the Register call can also call the new callback if `HID_API_HOTPLUG_ENUMERATE` is specified, `mutex_in_use` flag is set to prevent callback removal in that new callback.
8) The return value of any callbacks called for pre-existing devices is still ignored as per documentation and does not mark them invalid.