This does the following:

 - IV value is current 512-byte sector relative to start of loop
   container file.  This is what all cryptoloop people have done, if I
   am not mistaken.  Andi or others - if you can demonstrate the need
   for a more flexible setup an additional ioctl field may be needed.  I
   hope we can do without.

 - made some things static

 - made lo_offset a loff_t

 - added lo_sizelimit

   If one wanted a (crypto)loop somewhere inside a container file, the
   old code allowed a starting offset, but no size, so that the
   cryptoloop always extended to the end of the container file.  This
   field allows one to select an arbitrary interval.  Note that this
   changes struct loop_info64.

 - improve error handling of loop_init()

 - removed the unused typedef transfer_proc_t.

 - added a define for LO_CRYPT_CRYPTOAPI