[Rd] file.access() is unreliable in Windows (PR#7234)

[murdoch at stats.uwo.ca]

The file.access() function in base relies on the access() function in
the C library.  In Windows, that function only checks the directory's
permission bits for a file.   In versions of Windows post NT4, it's
possible to have access controls that change a user's access without
showing up in the directory record, and file.access() returns wrong
answers (e.g. if a user has read-only access to a network share, but a
particular file is not marked read-only).

I've changed the man page for file.access to record the fact that it's
results can't really be trusted on any multi-tasking system, and
especially on Windows, but I think a better fix should be made.  I
won't have time to do it until after 2.0.0.

Duncan Murdoch