r261913 broke DES passwords, because the only way they could work,
since they don't have an easily recognizable signature, was if they
were the default.  This commit rewrites crypt_set_format(3) etc to
address this:

 - Use a pointer instead of an index to identify the default format.
   This pointer is initialized at compile time to point to the first
   first element in the list of supported formats, eliminating the
   need for crypt_setdefault().  Using a pointer also simplifies
   iterating through the list.

 - Associate DES with the magic string "_", which takes care of the
   Extended DES format.

 - Finally, as a special case, if the salt does not match any known
   magic string but matches ^[./0-9A-Za-z]{13}$, it is assumed to be a
   DES password and is passed on to crypt_des().

MFC after:	1 week
Note that the bug was fixed, and when.
Add sysctl OIDs showing the actual size and capacity of the swap zone.

MFC after:	1 week