- Add a kthread to periodically call acctwatch() when accounting is active   instead of calling acctwatch() from softclock.  The acctwatch() function   needs to hold an sx lock and also makes a VFS call, and neither of these   are good things (or safe) to do from a callout.  The kthread only exists   and is running when accounting is turned on; it is started and stopped   as needed.  I didn't run acctwatch() via the thread taskqueue at Robert's   request as he was worried that if the accounting file was over NFS the   VFS_STAT() calls might stall other work on the taskqueue. - Add an acct_disable() function to take care of closing the accounting   vnode and cleaning up so we don't duplicate the same code in two   different places.
- Add a kthread to periodically call acctwatch() when accounting is active
  instead of calling acctwatch() from softclock.  The acctwatch() function
  needs to hold an sx lock and also makes a VFS call, and neither of these
  are good things (or safe) to do from a callout.  The kthread only exists
  and is running when accounting is turned on; it is started and stopped
  as needed.  I didn't run acctwatch() via the thread taskqueue at Robert's
  request as he was worried that if the accounting file was over NFS the
  VFS_STAT() calls might stall other work on the taskqueue.
- Add an acct_disable() function to take care of closing the accounting
  vnode and cleaning up so we don't duplicate the same code in two
  different places.

MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase