Interface: Rename Perdir to Suffix; provide Prefix

We want to be able to specify something that appears at the top of
every per-directory segment, principally to provide
for-every-directory variable settings.

A more sophisticated scheme akin to m4 diversions would be possible in
principle, but it would make organising the output files (which must
be processed by autoconf) really awkward - let alone the depenency
management for regenerating the makefiles.

So instead, simply provide a Prefix too.  Additionally, soon we are
going to introduce a Final.sd.mk which is processed right at the end.

I think this new scheme will be sufficient in principle to do anything
that might be needed.  Sadly it will continue to require that
&:include's are done in the right order.

"Suffix" and "Subdir" tab-complete really badly.  In a forthcoming
commit we will rename Subdir.sd.mk to Dir.sd.mk.

Incompatible change.  We must update the tests right away.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>