Make the buffer size for the perlio layer a macro.

Make the buffer size for the perlio layer a macro.

The 4K buffer size was chosen way back in bb9950b.  Significant performance
improvements are seen with larger buffer sizes, though the optimum size
likely varies by architecture and workload.  For starters, we'll leave the
default as-is but make the buffer size a macro and thus user-configurable via:

  sh Configure -Accflags=-DPERLIO_BUFSIZ=<number>

Choosing a better (larger) default is still TODO.