Revision 252310

- Add vnode-backed swap space specification support.  This is enabled when
  device names "md" or "md[0-9]*" and a "file" option are specified in
  /etc/fstab like this:

  md    none    swap    sw,file=/swap.bin       0       0

- Add GBDE/GELI encrypted swap space specification support, which
  rc.d/encswap supported.  The /etc/fstab lines are like the following:

  /dev/ada1p1.bde       none    swap    sw      0       0
  /dev/ada1p2.eli       none    swap    sw      0       0

  .eli devices accepts aalgo, ealgo, keylen, and sectorsize as options.

  swapctl(8) can understand an encrypted device in the command line
  like this:

  # swapctl -a /dev/ada2p1.bde

- "-L" flag is added to support "late" option to defer swapon until
  rc.d/mountlate runs.

- rc.d script change:

    rc.d/encswap -> removed
    rc.d/addswap -> just display a warning message if $swapfile is defined
    rc.d/swap1 -> renamed to rc.d/swap
    rc.d/swaplate -> newly added to support "late" option

These changes alleviate a race condition between device creation/removal
and swapon/swapoff.

MFC after:	1 week
Reviewed by:	wblock (manual page)