Make both iSCSI initiator and target support base64 encoded CHAP data.

While all tested initiators and targets use hex-encoded CHAP data, RFC also
allows base64 encoding there, and Microsoft certificaition tool uses it.

Reviewed by:	trasz (earlier version)
MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.

(cherry picked from commit 4fd3a4b49d761f4d722636a252840bdf7a0e0e2c)