MFC 245508,245566,245568,245611,245909:

MFC 245508,245566,245568,245611,245909:
Various fixes to timestamps in NFS:
- Use the VA_UTIMES_NULL flag to detect when NULL was passed to utimes()
  instead of comparing the desired time against the current time as a
  heuristic.
- Remove unused nfs_curusec().
- Use vfs_timestamp() to set file timestamps rather than invoking
  getmicrotime() or getnanotime() directly in NFS.
- Use NFSD_MONOSEC (which maps to time_uptime) instead of the seconds
  portion of wall-time stamps to manage timeouts on events.
- Remove unused nd_starttime from the per-request structure in the new
  NFS server.
- Use nanotime() for the modification time on a delegation to get as
  precise a time as possible.
- Use time_second instead of extracting the second from a call to
  getmicrotime().

git-svn-id: svn://svn.freebsd.org/base/stable/8@247503 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f