spa: document spa_thread() and SDC feature gates

spa_thread() and the "System Duty Cycle" scheduling class are from
Illumos and have not yet been adapted to Linux or FreeBSD.

HAVE_SPA_THREAD has long been explicitly undefined and used to mark
spa_thread(), but there's some related taskq code that can never be
invoked without it, which makes some already-tricky code harder to read.

HAVE_SYSDC is introduced in this commit to mark the SDC parts. SDC
requires spa_thread(), but the inverse is not true, so they are
separate.

I don't want to make the call to just remove it because I still harbour
hopes that OpenZFS could become a first-class citizen on Illumos
someday. But hopefully this will at least make the reason it exists a
bit clearer for people without long memories and/or an interest in
history.

For those that are interested in the history, the original FreeBSD port
of ZFS (before ZFS-on-Linux was adopted there) did have a spa_thread(),
but not SDC. The last version of that before it was removed can be read
here:

  https://github.com/freebsd/freebsd-src/blob/22df1ffd812f0395cdb7c0b1edae1f67b991562a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa.c

Meanwhile, more information on the SDC scheduling class is here:

  https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/common/disp/sysdc.c

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #15406