Add funopen() function

This is a wrapper over the glibc fopencookie() function.

We diverge from the FreeBSD, OpenBSD and DragonFlyBSD declarations,
because seekfn() there wrongly uses fpos_t, assuming it's an integral
type, and any code using that on a system where fpos_t is a struct
(such as GNU-based systems or NetBSD) will fail to build. In which case,
as the code has to be modified anyway, we might just as well use the
correct declaration.

src/libbsd.map

@@ -110,3 +110,7 @@ LIBBSD_0.6 {
     /* Exported to cope with the constructor+dlopen+threads mess. */
     setproctitle_init;
 } LIBBSD_0.5;
+
+LIBBSD_0.7 {
+    funopen;
+} LIBBSD_0.6;