Fix t2124 - node variable should try to access a file outside of tup.

Just using ".." wasn't going quite far enough, so using "../foo.txt" will actually end up setting dt to 0 in get_tent_dt(). We need to check for the 0 return value so we can return an error, rather than call tup_entry_get() on a 0 tupid.
Added t2132 - node variables can't refer to deleted generated nodes.

Similar to other things that handle generated nodes, we have to make sure that when they are in the delete list, they can't be used during parsing. Otherwise an incremental build may behave differently from a full build.