Remove residual blank line at start of Makefile

This is a residual of the $FreeBSD$ removal.

MFC After: 3 days (though I'll just run the command on the branches)
Sponsored by: Netflix

sys: Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

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 change

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

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