When building the expansion tree, distinguish between apparently
identical nodes that appear in different contexts.  The same delivery
can happen for different users, or as a different destination address.

Fixes a bug with setups like the following, where a mail sent to
contact@foo.net was not delivered to both /tmp/me and /tmp/she.

table foobar { me@foo.net = vmail, she@foo.net = vmail, \
	       contact@foo.net = "me@foo.net,she@foo.net" }
accept for domain foo.net virtual <foobar> \
	deliver to maildir "/tmp/%{dest.user}"