Allow initiator to verify target authentication using CHAP.

Return the most appropriate of EACCES, EPERM, ENODEV, ENOTSUP, EIO or
EINVAL depending on the exact error returned by the target, rather than
just always returning EPERM.

Also, ensure that error strings exist for these errors.

really ugly hack at present, but that doesn't hugely matter since I'm
aiming to change the interface to iSCSI devices anyway within the next
week.

SCSI interface not yet implemented.

will enable us to cascade async operations, which is necessary in order to
properly support DNS.  (For example, an HTTP request may have to redirect
to a new location and will have to perform a new DNS lookup, so we can't
just rely on doing the name lookup at the time of parsing the initial
URL).

Anything other than HTTP is probably broken right now; I'll fix the others
up asap.

TargetAddress from the login response, because we still need the old
address while we close the connection!

Added CHAP authentication

negotiation phases, to allow inclusion of CHAP authentication.

errors; still need to verify data integrity).

SCSI response PDUs are handled: status and sense data (if available) are
returned via the scsi_command structure.

Updated iSCSI session parameter usage.

declaration.

I want to get to the point where any header in include/ reflects a
standard user-level header (e.g. a POSIX header), while everything that's
specific to gPXE lives in include/gpxe/.  Headers that reflect a Linux
header (e.g. if_ether.h) should also be in include/gpxe/, with the same
name as the Linux header and, preferably, the same names used for the
definitions.

blocks.