MFC r257151:

  Document /var/cache/pkg into hier(7) which pkg(8) uses.

Approved by:	bapt
MFC r257206:
Fix a deadlock when trying to power off a USB device. The deadlock
happens because the code in question is trying to modify the parent
USB port registers outside the USB explore thread.
MFC r257221:
Fix typo.
MFC r257154:

Merging local and remote bitmaps must be protected by hr_amp lock.

This is believed to fix hastd crashes, which might occur during
synchronization, triggered by the failed assertion:

 Assertion failed: (amp->am_memtab[ext] > 0),
 function activemap_write_complete, file activemap.c, line 351.