(last_node): Make it a global variable.

(last_node): Make it a global variable.
(process_file): Print the tags from the nodes as soon as
possible, and delete the nodes.  This brings down the memory
occupancy as etags to almost the same level as when the #line
directives were not parsed.
(free_fdesc): New function.
(find_entries): Use it.
(invalidate_nodes): In etags mode, do not just mark the nodes as
invalid, do delete them.