Fix Mercury support, notably qualified procedures.

Fix Mercury support, notably qualified procedures.

    Correct the previous fix (did not correctly handle qualified types).
    Also fix the following issues:
    - remove module name (+ dot) from tags, as prefixing module name is
      often inconsistent in code and may cause tags to be too specific.
    - now tag 0-arity predicates and functions (':- func foo_14.')
    - now tag one-word declarations (':- interface.')

    * lib-src/etags.c (mercury_pr): Pass the correct NAME and NAMELEN
    arguments to 'make_tag'.
    (mercury_decl): Return more information about the declaration or
    definition it finds.  This allows mercury_pr to be smarter.
    (Bug#47408)