loader: Fix typos in help.common

- ahci(4) doesn't use .0 for disable hint
- fix slice name in example
loader: Fix XHCI description in help.common
Import cpuctl pseudo device from FreeBSD

The cpuctl pseudo device allows to perform cpu microcode updates.
drm: Use lockmgr locks with Linux wait queues

* On Linux, it is possible to grab a second spinlock in an already
  spinlock-protected code section.

* The wait_event() macro is used in such a situation in the i915 driver.
  One of the event checks itself tries to grab a second lock. What's more,
  this second lock is a lockmgr lock in the DragonFly kernel. This results
  in the following situation:

    spinlock
    lockmgr LK_EXCLUSIVE
    spinunlock

* Unfortunately if the lockmgr lock can't be acquired, the thread is put
  to sleep and a thread sleeping with a spinlock held leads to a general
  system freeze or a kernel panic.

* For that reason, we can't use a spinlock in Linux wait queues. Change
  the internal wait_queue_head_t lock to a lockmgr lock.

Thanks go to Imre Vadász for spotting this horrible issue.
cpuctl: Fix path in 'files'.
vn_lock.9: Capitalize .Dt
hammer - Better trailing '/' handling on PFS paths

- Remove all trailing slashes from PFS paths to make sure
  there are no problems when performing operations on them.
cpuctl.4: Some small fixes.
usbdi.9: Some small fixes.
svc.8: Fix a number of references to other manual pages.
FWIW, bring in FreeBSD's rgephy(4) manual page.

flag0 support has been commented out since I could not find it in our
code.