swapon - Implement encrypted swap

swapon - Implement encrypted swap

Implement crypting of the swap device.  When enabled in this manner
/dev/urandom is used to generate a 256-bit random key and the base device
is automatically cryptsetup and mapped, making crypted swap trivial.

* Implement the 'crypt' fstab option, so swapon -a and swapoff -a work
  as expected for crypted swap.  Again, the base device (e.g. /dev/da0s1b)
  should be specified.  The option will automatically map it with cryptsetup
  and swap on the mapping.

  Also implement -c to crypt manual swapon/swapoff commands.  If used for
  swapon it must also be used for swapoff.  Again, specify the base device
  (e.g. /dev/da0s1b), not the /dev/mapper device, for both cases.

* Implement the 'trim' fstab option, allow trim to be specified in the fstab
  instead of requiring a rc.conf option.

* The trim option no longer exits with an error if the device does not
  support TRIM or TRIM is not enabled.