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>

dive.h

@@ -463,7 +463,7 @@ const char *monthname(int mon);
 extern const char *star_strings[];
 
 extern const char *default_filename;
-extern char *existing_filename;
+extern const char *existing_filename;
 extern const char *subsurface_default_filename(void);
 #define AIR_PERMILLE 209