hammer2 - Reduce flush complexity, fix panics

* Set synchronous flush mode by default for now to reduce sources of
  potential crashes so the core can be stabilized.

  This means that the only transactions that will run with a higher
  transaction id during a flush are allocations from the freemap made
  by the flush itself.  This is far easier to debug.

  Concurrent frontend transactions are disabled during a flush, for now.

* The buffer flush which is run synchronously from the main flush now always
  uses an async + sync vfsync sequence and does not allow any buffers to
  be left over before entering the main flush.

* The buffer flush now also uses the same transaction id as the main flush.
  A new flag, HAMMER2_TRANS_PREFLUSH is used to manage this mechanic and
  to assert on any illegal buffer flushes.

* Fix miscellanious assertions that were incorrect, and split the pass4
  codepath in the flush into pass4 and pass5 to fix another bug.

* Fix an improper semicolon (from swildner)

* Remove more dead code.

* The hammer2_vop_reclaim() code was the only code path that would acquire
  a hammer2_inode lock prior to starting a transaction for a modifying
  operation (for the delete unlinked file on last reference).  Fix this
  code to open the transaction first and obtain the inode lock second,
  which fixes a deadlock.

* Fix an infinite while() loop due to improper arguments in a TAILQ_NEXT()
  call.
hier.7: Add /compat/ndis, binutils adjustments, remove /etc/firmare.
Add a urtwnfw(4) manual page (from FreeBSD).
Add an /etc/periodic script to check for dports package vulnerabilities.

It's basically just 'pkg audit' at them moment, the output of which is
then included in the nightly security mails.

To disable this check for some reason, daily_status_dports_audit_enable
can be set to 'no' in /etc/periodic.conf.

Reported-by: joris
Revert "Add an /etc/periodic script to check for dports package vulnerabilities."

This reverts commit dced1188f0aae7a3a714a58540f8f6bf4043f35c.

Aw, actually there are scripts in /usr/local/etc/periodic doing just that.