initial import of hidapi netbsd uhid native backend from mainstream

c19ae126d8
https://github.com/libusb/hidapi/pull/612

src/hidapi/netbsd/README.md

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+Implementation Notes
+--------------------
+NetBSD maps every `uhidev` device to one or more `uhid`
+devices. Each `uhid` device only supports one report ID.
+The parent device `uhidev` creates one `uhid` device per
+report ID found in the hardware's report descriptor.
+
+In the event there are no report ID(s) found within the
+report descriptor, only one `uhid` device with a report ID
+of `0` is created.
+
+In order to remain compatible with existing `hidapi` APIs,
+all the `uhid` devices created by the parent `uhidev` device
+must be opened under the same `hid_device` instance to ensure
+that we can route reports to their appropriate `uhid` device.
+
+Internally the `uhid` driver will insert the report ID as
+needed so we must also omit the report ID in any situation
+where the `hidapi` API expects it to be included in the
+report data stream.
+
+Given the design of `uhid`, it must be augmented with extra
+platform specific APIs to ensure that the exact relationship
+between `uhidev` devices and `uhid` devices can be determined.
+
+The NetBSD implementation does this via the `drvctl` kernel
+driver. At present there is no known way to do this on OpenBSD
+for a `uhid` implementation to be at the same level as the
+NetBSD one.