Tweak original coding so that we can determine the platform-specific

Tweak original coding so that we can determine the platform-specific
shared_buffers and max_connections values to use before we run the
bootstrap process.  Without this, initdb would fail on platforms where
the hardwired default values are too large.  (We could get around that
by making the hardwired defaults tiny, perhaps, but why slow down
bootstrap by starving it for buffers...)