srogue: add and use md_fdopen().

The implementation was copied from rogue4.  Using fdopen() is necessary
because the scorefile needs both encread() and encwrite().  For some
reason I have failed to discover, one of them uses FILE *'s and the
other uses file descriptors.

srogue/rogue.h

@@ -40,6 +40,7 @@ int	md_chmod(const char *filename, int mode);
 char	*md_crypt(const char *key, const char *salt);
 int	md_dsuspchar(void);
 int	md_erasechar(void);
+FILE *	md_fdopen(int fd, char *mode);
 char	*md_gethomedir(void);
 char	*md_getusername(void);
 uid_t	md_getuid(void);