Update the regression test harness to support VMS.

Update the regression test harness to support VMS.

* config_flags_pm.com, test_make.com: set up and run the regression
test environment on VMS.
* tests/run_make_tests.pl [VMS]: Use an alternate rmdir()
implementation on VMS.
(run_make_with_options) [VMS]: Provide VMS-specific quoting and shell
invocations.
(set_more_defaults) [VMS]: Set default values when running on VMS.
* tests/test_driver.pl (vms_get_process_logicals) [VMS]: Retrieve the
proper values from %ENV on VMS.
(resetENV) [VMS]: Use it.
(toplevel) [VMS]: Fix a bug with opendir() on some logical_devices.
(compare_output) [VMS]: Convert VMS test output to a "standard" format.
(_run_command) [VMS]: Handle signals and exit codes the VMS way.
(remove_directory_tree_inner) [VMS]: Unlink all versions of the file.