switches passed from the Makefile.  This looks like it will fix
problem with virtual vs real paths under msys.

just exec instead of creating a subprocess.  This reduces process usage
from four processes per parallel test to two.  I have no idea whether
a comparable optimization is possible or useful in the Windows port.

'make install' or 'initdb' fails.  Also minor simplification of fgets()
usage --- fgets guarantees a trailing null anyway.

This allows it to be used on Windows without installing mingw
(though you do still need 'diff'), and opens the door to future
improvements such as message localization.
Magnus Hagander and Tom Lane.