file ccaches and stdio

[Ken Raeburn]

The MIT file ccache code has support for both file descriptor
(POSIX-y) and stdio (ANSI C) access to the file system.

As far as I can tell, the file descriptor code is going to get used on
any platform that the rest of our code will build on.  In our source
tree, it gets used if HAVE_SYS_TYPES_H is defined, which happens on
Windows, and should happen on any UNIX system that has that header
file, and we have other source files that just include <sys/types.h>
unconditionally.

Is anyone using the stdio code, or should I just scrap it and simplify
the maintenance?

Ken