Fix libstand build breakage after r361298.

Fix libstand build breakage after r361298.

- Use enc_xform_aes_xts.setkey() directly instead of duplicating the code
  now that it no longer calls malloc().
- Rather than bringing back all of xform_userland.h, add a conditional
  #include of <stand.h> to xform_enc.h.
- Update calls to encrypt/decrypt callbacks in enc_xform_aes_xts for
  separate input/output pointers.

Pointy hat to:	jhb