Once again improve existing filename handling

Several potential problems.
- we could end up dereferencing exiting_filename when it was NULL
- we could free the default_filename by mistake -
  subsurface_default_filename always needs to return a copy of it
- closing the existing file before opening a new one repopulated the
  existing_filename with the default filename - preventing the opened
  file to become the new existing filename

Also, make existing filename a const char * and make file_open have the
same sensible default folder behavior as the other file related functions.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

windows.c

@@ -98,7 +98,7 @@ const char *subsurface_icon_name()
 const char *subsurface_default_filename()
 {
 	if (default_filename) {
-		return default_filename;
+		return strdup(default_filename);
 	} else {
 		char datapath[MAX_PATH];
 		const char *user;