Allow VMS features to differ from Perl's defaults.

Since we now enable some features that are not the CRTL's defaults,
we should allow them to be explicitly disabled in the environment
if someone wants the old behavior.
Implement useversionedarchname in configure.com.

If this ever becomes the default, we should really remove the
versioned subdirectory from archlib that we've always had.
As of now, using this feature on VMS gives us redundancies such as:

archlib='perl_root:[lib.VMS_IA64-5_17_9.5_17_9]'