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