risc-v: add ofctl(8) and /dev/openfirm support

branches: 1.9.6;
RISC-V support that works on QEMU with a single hart.

Thanks for Simon Burge for plic(4).

Trailing whitespace

Systematize handling of removed drivers.

- Every driver that was removed and whose number hasn't already been
reused is now listed with a commented-out "obsolete" line.
- The format of these has been systematized. Future format changes can
probably be safely done with a script.
- This does not include a few cases of assignments that only lasted a
couple days, or stuff from before major reorgs. Some of these may
be included nonetheless, because there was a lot of ground to cover
and therefore not a lot of time to dig into history in detail.

Note that the obsolete listings do not mean the major numbers can
never be reused; that's up to portmasters and/or core. It does mean
that they won't be reused by accident, however, which in some cases
(depending on the driver, how widely used it was, its family of device
nodes, their default permissions, etc.) can be quite dangerous.

Note that some of the things now explicitly listed as obsolete are
really ancient history. My scan went back as far as when the majors
files were added. (But not before that.)

This may have been cutpasted from evbmips, but we don't need to say so.

Restore satlink's majors entries commented out and marked obsolete.
Otherwise they might accidentally get reused later and cause a
security problem.

Remove the satlink driver. It was disabled everywhere, had no man page and
no use either. Spotted by thorpej in PR/21345, ok christos.

Remove ISDN from the kernel. It has remained unmaintained for a long time,
is of poor quality, and is now an obstacle to MP-ification. It was removed
ten years ago from FreeBSD for the same reason.

This retires a big user of the mbuf API, and will ease maintenance of the
kernel.

branches: 1.2.14; 1.2.16; 1.2.18;
Add a synthesized pc beeper and keyboard bell for platforms with an audio
device.

branches: 1.1.2; 1.1.4;
New files for Userland support of UCB RISC-V (both 32-bit and 64-bit)