Call modules iteratively, rather than recursively.

	Call modules iteratively, rather than recursively.

	This cleans up duplicate code that was in modcall.c, and means
	that it's more likely we can put proxy into a module in the
	authenticate section, in order to have it part of configurable
	failover.

	It also means that the "proxy EAP-MSCHAPv2 as MS-CHAPv2" and related
	work will become easier, as we don't have to walk back up the entire
	stack.  Instead, we can just keep the "modcall stack" associated
	with the request, and continue where we left off.