Added CHAP authentication

away code such as

  if ( strcmp ( some_string, "some_value" ) != 0 ) {
     DBG ( "Bad value received!" );
  }

for realloc(0) being a valid way to free memory.

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.

with octal yet, but we can probably live without that.

negotiation phases, to allow inclusion of CHAP authentication.

while retaining the format string checking.

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.