ipfw(8)/kldconfig(8): Clean up some __FreeBSD_version checks.
libopie: Update config.h, clean up Makefile and use CSTD=gnu99.

* Include all necessary includes to compile with -std=gnu99, our default.

* Add some missing prototypes to <opie.h>.

* Drop login.c and logwtmp.c from the build. Also drop libmissing/* files.

* Update config.h for <utmpx.h>, getutxline() and pututxline().
kernel - Fix vn_syncer_add panic with UFS

* The new VONWORKLST requirements were expanded to include VOBJDIRTY
  for both the standard syncer and the new syncer.  There was a path
  in the VM system which could clear VOBJDIRTY but failed to resynchronize
  VONWORKLST which could cause UFS to accidently leave a vnode on the
  syncer list after reclamation, causing a crash.

* Fix the VM path.

Reported-by: David Shao