Various fixes for the encrypted storage support.

Various fixes for the encrypted storage support.

- create_initial_config_files() and node_write_config() are now the only
  functions that generate the content of new config files from scratch.
- All public API functions that change config files now immediately
  write them out.
- Config files of nodes that join using an invitation file are immediately
  written out.
- Ensure nodes marked dirty have their config files written out in
  periodic_handler(), and on meshlink_stop().
- Fix some memory leaks.
- Write out updated config files, and recreate mesh->self in meshlink_set_port().