Add external_acl_type %EXT_LOG and %EXT_TAG format options.

Add external_acl_type %EXT_LOG and %EXT_TAG format options.

%EXT_LOG and %EXT_TAG are filled with the log= and tag= fields
returned by previous external ACLs.

-for a string that never changes after it is set, tag= is suitable.
-for a string that may need updating or overwriting by a later
external_acl, log= is suitable.

Under both circumstances it is conceivable that later external_acls
may need access to the tag= or log= values after they have been set
(e.g. for external_acl debugging, merging log messages, etc).