Simplify uses of readdir() by creating a function ReadDir() that

Simplify uses of readdir() by creating a function ReadDir() that
includes error checking and an appropriate ereport(ERROR) message.
This gets rid of rather tedious and error-prone manipulation of errno,
as well as a Windows-specific bug workaround, at more than a dozen
call sites.  After an idea in a recent patch by Heikki Linnakangas.