kldload(8): Add -n for checking if a module is already loaded.

Also, give better error messages upon EEXISTS and ENOEXEC.

Taken-from: FreeBSD
rc: Use kldload -n in our rc scripts which load modules.
usb4bsd: Fix some mistakes i made in 5782e8a131ef79.
rc.subr: Add get_pidfile_from_conf (from FreeBSD).
devd(8): Sync with FreeBSD and turn it on by default.

Most important of all, it cares about knowing which USB modules
support which device/vendor IDs and it will load the associated
module upon USB device plugging if it is not already loaded or
in the kernel config.

Taken-from: FreeBSD
Add a tool to keep the devd(8) database in sync with our USB device support.

Taken-from: FreeBSD
bus_autoconf.sh: Stop putting $FreeBSD$ into the usb.conf database.

It is generated locally and gets never synched with FreeBSD.
usb4bsd: Remove usbdevs_data.h again.

It is automatically generated and only usbdevs needs to be modified for
new IDs.

Reported-by: tuxillo