Fix handling of linked rename chains in incremental backups

Fix handling of linked rename chains in incremental backups

* src/incremen.c: Change the meaning of the DIRF_RENAMED flag.  Now it
marks a directory which is the last one in a chain of renames.
Regular renamed directories are recognized by their orig member being
non-NULL.  Directories marked with DIRF_RENAMED start encoding of renames.
(procdir): Clear DIRF_RENAMED flag on directories which are origins for
renames.
(makedumpdir): Use the orig member to check if the directory is a
result of a rename.
(store_rename): Move the check for DIR_IS_RENAMED to the caller. Don't
clear the DIRF_RENAMED, it is not needed any more.

* tests/rename06.at: New test.
* tests/Makefile.am: Add rename06.at
* tests/testsuite.at: Likewise.