...for checking compatibility with the reference rdbs.  unoidl-check is no
longer based on the legacy registry format, but can process all the various new
UNOIDL registry formats.  regcompare is still included in the SDK for now.

(gb_UnoApi[Target]_set_reference_rdbfile now takes a non-empty sequence of rdb
files, any necessary dependencies of the final rdf file preceding it just like
it is required on the unoidl-check command line.  Also, executing the
unoidl-check now properly depends on those rdb files.)

TODO: unoidl-check is too conservative for now and flags some changes as
incompatible that are not.

Change-Id: I92e4c69403c5e3fcb31707c98c65a2f509592dd4