listmgr: deal with partial paths in DB.

It can happen that the parent directory for an entry is not
in NAMES table, so the path built by one_path() or this_path()
stored functions is partial and is not relative to FS root.
This patchs check that the built path is under FS root
by checking the id of the top level directory that was used
to build the path.

Adapted filter and conversion functions to deal with that.
backup mode: properly manage archiving and undelete of entries with partial path.

If the path stored in the DB is partial (parent directory missing in the DB)
the entry is migrated to a special "__unknown_path" directory in the backend.
Also manage undelete correctly for those entries.
Added test 115 to test such cases.
tests: fix test 115 for lhsm (doesn't manage backend path).