(ada-gnatls-args): New variable. Add support for specifying arguments to

(ada-gnatls-args): New variable.  Add support for specifying arguments to
gnatls.
(ada-initialize-runtime-library): Properly parse "." in the output of gnatls.
(ada-add-keymap): Removed, since this is now done in ada-mode.el itself.
(ada-add-ada-menu): Likewise.
(ada-set-default-project-file): New parameter KEEP-EXISTING.
(ada-prj-find-prj-file): New parameter FILE.
(ada-parse-prj-file): Take into account the ADA_INCLUDE_PATH and
ADA_OBJECTS_PATH environment variables.  Minor reorganization of the code
(ada-get-all-references): Add support for GNAT 3.16 cross-references.