MFC 273644,273738:
Clarify that pthread_cleanup_push()/pop() are implemented as macros that
create a new code block and thus must be balanced at the same lexical
scope.  (This is also a requirement in POSIX.)

PR:		194280
Submitted by:	dr2867.business@pacbell.net
MFC 273834:
Rework the EXAMPLES section to be a bit clearer.
- Add an example of using etcupdate diff.
- Create a subsection on bootstrapping that is below the simple
  examples.  This should make it clearer that 'etcupdate extract' is
  a one-time operation and not part of the common workflow.  It also
  adds more suggestions on when bootstrapping is needed and additional
  steps to make future merges simpler.
MFC 273598 273602 273607 273613 273647:

    Userland HPET support.
MFC r273711:
Allocate buffer for READ BUFFER/WRITE BUFFER commands on demand.

These commands are rare, but consume additional 256KB RAM per LUN.
MFC r273730, r273731:
Reduce code duplication around Write Exclusive persistent reservation.

While there, allow some more commands to pass persistent reservation.
MFC r273809:
Implement better handling for ENOSPC error for both CTL and CAM.

This makes VMWare VAAI Thin Provisioning Stun primitive activate, pausing
the virtual machine, when backing storage (ZFS pool) is getting overflowed.