add cleaning of temporary test files (with option to keep them)

add cleaning of temporary test files (with option to keep them)

    default.mk: add $(testdir) definition based on $(TMPDIR), which
    defaults to "/tmp" if not already defined

    targets.mk(check): call test suite with TMPDIR=$(testdir) and remove
    direactory after successful run of testsuite.  Do not remove
    temporary test files if $(TEST_NO_AUTOCLEAN) is set to a non-empty
    value.

    targets.mk(cleandirs): refactor the first part of what cleanall
    had been doing.

    targets.mk(cleanall): run cleantest and cleandirs, then remove
    backup files.

    targets.mk(cleanlisp): remove backup files.

    targets.mk(cleandoc): remove backup files.

    targets.mk(cleanall): run cleantest and cleandirs, then remove
    backup files.

    targets.mk(cleantest): removal of temporary test files.