Summary: Make aclCheck_t a class (ACLChecklist), and hopefully fix bug 516.

Summary: Make aclCheck_t a class (ACLChecklist), and hopefully fix bug 516.
Keywords:

	Extract aclCheck_t from structs.h to ACLChecklist.h, and
	rename to ACLChecklist.
	Add ACLChecklist to the SQUID_sources to have it distributed.
	Remove aclCheck_t as a global cbdata, instead use per class
	cbdata.
	Create constructor and destructors for ACLChecklist.
	Remove all aclCheck_t memset's.
	Make aclCheckCallback into ACLChecklist::checkCallback.
	In clientCheckNoCache, cbdatareference the context before handing off to
	aclNBCheck.
	In clientCheckNoCacheDone, check that http is still valid before using it.