hid: Import usbhid - USB transport backend for HID subsystem.

This change implements hid_if.m methods for HID-over-USB protocol [1].

Also, this change adds USBHID_ENABLED kernel option which changes
device_probe() priority and adds/removes PnP records to prefer usbhid
over ums, ukbd, wmt and other USB HID device drivers and vice-versa.

The module is based on uhid(4) driver. It is disabled by default for
now due to conflicts with existing USB HID drivers.

[1] https://www.usb.org/sites/default/files/hid1_11.pdf

Reviewed by: hselasky
Differential revision: https://reviews.freebsd.org/D27893