as of version 4.0 (april 88), the follwing need to be done to track:
bugs:
   -d option doesn't work with non-default sub-list/statfile names. (FIXED)
   -w makes statfile with root 644 protections.	(not clearly a bug).

1) fix nopullflag bug (described in 4.0 checkin message).
   if the importing dir-name (say /bin) doesn't exist,
   every subordinate file-creation complains of the missing entry-dir.
   the updated() call in update_file() needs more subtlety for this case.

2) add @ flag for exceptions:

   in general, "@name" would mean,
   "export/import name's target", if name is a link,
   or "export/import name", otherwise.
   if name's ultimate target is a dir, that dir will be exported normally;
   its links will not be traced.

   "->@name" would mean, export/import a link to name's target.
   note that the subscriber's link will point to the exporting file-system.

3) add "backup" option, which ensures that the importing dir's are
   up-to-date with respect to the exporting dir. no statfile is used.
   this can be implemented as a variant of writestat's traversal.

4) add -C (comparison-root) option.

5) remove -u option, which is now the default.
   add the opposite option, preventing updates of newer files.

6) add -E option, for adding temporary global exceptions.
7) add Ralph's syntax for temporary entry-exceptions.
