Allow initiator to verify target authentication using CHAP.

wasn't meant for my local "master" branch ;)
This reverts commit 373022108b.

Don't crash when called on an uninitialised chap structure; this
allows us to avoid extra checks within iscsi.c to make sure that we
receive the CHAP_XXX keys in a sensible order.

code, rather than going through the digest layer.

Removed a spurious line of constants.

defined in vsprintf.h.  (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)

There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.