- Modify swapon(8) so that it uses most of geli(8) defaults for swap,
   which is presently: AES-XTS, no authentication.  Create provider
   with pagesize as sectorsize by default.
 - Rewrite parsing code for geli(8)-backed swap options, now options
   are required to be exact match, and unrecognized options will trigger
   a warning.
 - Don't initialize GELI device if it's already initialized.  This
   restores previous behavior.
 - Don't duplicate file descriptor when working with geli(8) and
   gbde(8) as there is no need to communicate with the utility other
   than exit status.
 - When calling swap_on_off_* routines, which_prog can only be SWAP_ON
   or SWAP_OFF.  Eliminate unneeded case branches by replacing switch
   with if's.
 - Plug a few memory leaks.

Reviewed by:	hrs (but bugs are mine)
MFC after:	1 week
X-MFC-with:	r252310, r252332, r252345