Start rationalizing Unix-to-VMS file spec conversion code.

Start rationalizing Unix-to-VMS file spec conversion code.

Back in 360732b5267d5dfef32b932bf13ceebc6683df74, we started using
an experimental new conversion routine that had been designed for
a CRTL feature called POSIX-compliant pathnames but at this point
was added as a jumping-off place halfway through the existing code
for converting Unix-format file specifications to VMS format. But
only for newer versions of VMS and only when a different and
unrelated feature called Extended Filename Syntax (EFS) had been
enabled.

But this newer implementation (somewhat inauspiciously named
posix_to_vmsspec_hardway) is less complete and more buggy than the
older implementation, and it imposes expectations that have nothing
to do with EFS, not to mention making for a larger, version-
specific support matrix.

So for now go back to the older, better-tested (though imperfect)
version and simplify the differences made by invoking EFS.  None
of this makes any difference at all unless non-default CRTL
features have been enabled.