Cygwin: improve O_TMPFILE handling

Windows does not remove FILE_ATTRIBUTE_TEMPORARY by itself after a
file has been closed.  It's just some attribute which can be set or
removed at will, despite its purpose.

Apparently there are tools out there which use FILE_ATTRIBUTE_TEMPORARY
accidentally or wrongly, even Microsoft's own tools are affected.  In
the end, the filesystem is potentially full of files with this attribute
set.

Implement O_TMPFILE files with FILE_ATTRIBUTE_TEMPORARY and
FILE_ATTRIBUTE_HIDDEN set.  This combination is pretty unlikely.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.h

@@ -37,6 +37,8 @@ details. */
    never be used in Cygwin for this function. */
 #define PIPE_ADD_PID	FILE_FLAG_FIRST_PIPE_INSTANCE
 
+#define O_TMPFILE_FILE_ATTRS (FILE_ATTRIBUTE_TEMPORARY | FILE_ATTRIBUTE_HIDDEN)
+
 extern const char *windows_device_names[];
 extern struct __cygwin_perfile *perfile_table;
 #define __fmode (*(user_data->fmode_ptr))