IMPORT: cebtree: import version 0.5.0 to support duplicates

IMPORT: cebtree: import version 0.5.0 to support duplicates

The support for duplicates is necessary for various use cases related
to config names, so let's upgrade to the latest version which brings
this support. This updates the cebtree code to commit 808ed67 (tag
0.5.0). A few tiny adaptations were needed:
  - replace a few ceb_node** with ceb_root** since pointers are now
    tagged ;
  - replace cebu*.h with ceb*.h since both are now merged in the same
    include file. This way we can drop the unused cebu*.h files from
    cebtree that are provided only for compatibility.
  - rename immediate storage functions to cebXX_imm_XXX() as per the API
    change in 0.5 that makes immediate explicit rather than implicit.
    This only affects vars and tools.c:copy_file_name().

The tests continue to work.