• 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:
  1. 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)