Windows: Implement a wrapper of the open() function.

The wrapper does two things:
- Requests to open /dev/null are redirected to open the nul pseudo file.
- A request to open a file that currently exists as a directory on
  Windows fails with EACCES; this is changed to EISDIR.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

compat/mingw.h

@@ -137,6 +137,9 @@ int sigaction(int sig, struct sigaction *in, struct sigaction *out);
  * replacements of existing functions
  */
 
+int mingw_open (const char *filename, int oflags, ...);
+#define open mingw_open
+
 char *mingw_getcwd(char *pointer, int len);
 #define getcwd mingw_getcwd