u2f(4): a HID driver for FIDO/U2F security keys

While FIDO/U2F keys were already supported by the generic uhid(4) and
hidraw(4) drivers, this driver adds some additional features an does
steps to ti

u2f(4): a HID driver for FIDO/U2F security keys

While FIDO/U2F keys were already supported by the generic uhid(4) and
hidraw(4) drivers, this driver adds some additional features an does
steps to tighten the security of FIDO/U2F access.

- It automatically loads through devd.
- Automatically enables HQ_NO_READAHEAD for FIDO/U2F devices.
- Implements only miminum set of features.
- Do not requires external devfs configuration to set character device
permissions.
- Names character device as u2f/# to make possible capsicum or any
other pledge()-style sandboxing.

PR: 265528
Differential Revision: https://reviews.freebsd.org/D51612

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