refactor: roster export uses GString and g_file_set_contents now

Modernize the /export command implementation by adopting GLib-based
patterns for string manipulation and file handling.

* Replace _writecsv with _append_csv_escaped to efficiently handle CSV
  quote doubling directly within a GString buffer
* Refactor cmd_export to build the CSV content in memory using GString,
  minimizing system calls
* Use g_file_set_contents for atomic writing

src/command/cmd_funcs.c

@@ -1074,7 +1074,6 @@ cmd_script(ProfWin* window, const char* const command, gchar** args)
     return TRUE;
 }
 
-/* escape a string into csv and write it to the file descriptor */
 static void
 _append_csv_escaped(GString* buffer, const char* const str)
 {