libiscsiutil: Change keys_load/save to operate on data buffers.

This will be used in future changes to support large text requests
spanning multiple PDUs.

Provide wrapper functions keys_load/save_pdu that operate use a PDU's
data buffer.

Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D33547

(cherry picked from commit 25700db36640b1538ac91f893955a4f1a4167f63)