Low: RA: iSCSITarget: rename CHAP authentication parameters, make username unique

Since CHAP authentication, as presently implemented in the RA, only
applies to "incoming user" authentication, rename parameters
accordingly:

• "username" -> "incoming_username"
• "password" -> "incoming_password"

This will allow us to add support for outgoing user authentication
without breaking compatibility.

Also, since CHAP authentication accounts are target specific in only
some iSCSI target implementations, make usernames unique. If in an
iSCSI implementation with no per-target user accounts the same
username were used for multiple targets, they would all map to one
account, with passwords overriding one another.