Return a more accurate error when can't open a unique file.
Return a more accurate error when can't open a unique file.

Jason Rust <jrust@rustyparts.com> reported a problem where
svn_io_open_unique_file() claimed "Ran out of unique names" when the
real problem was permissions.  This behavior was introduced in
revision 6838, which solved issue #1487 ("svn diff -r PREV:HEAD fails
if tmp/ exists in current directory") by treating APR_EACCES the same
as APR_EEXIST, due to a Windowsism whereby EACCES results when you
open a file on top of a directory.

* subversion/libsvn_subr/io.c
  (svn_io_open_unique_file): If got APR_EACCES when creating the file,
    then see if we've encountered the aforementioned Windowsism, and
    either continue iterating or return error as appropriate.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion