Make file-existence test a per-platform function.

NFC in this commit, but this will allow me to do something more subtle
and OS-specific in each OS's implementation of it.

putty.h

@@ -1467,6 +1467,7 @@ Filename *filename_deserialise(void *data, int maxsize, int *used);
 char *get_username(void);	       /* return value needs freeing */
 char *get_random_data(int bytes, const char *device); /* used in cmdgen.c */
 char filename_char_sanitise(char c);   /* rewrite special pathname chars */
+int open_for_write_would_lose_data(const Filename *fn);
 
 /*
  * Exports and imports from timing.c.