Config.update_config() applies submitted options to the main
configuration file via configparser read-modify-write: unknown
options are preserved, null removes an option, the merged view
of all files is validated as ServerConfig before anything is
written (a bad file would kill the FileWatcher polling loop),
and the write is atomic (.tmp + os.replace, mode 0600). Options
whose effective value is owned by a later configuration file
raise ConfigConflictError instead of writing a no-op. The
reload logic is factored into reload_and_notify() and the file
watcher callback is exception-guarded so polling never dies.
This monitor for change the file from dynamips by computing a
hash of the watched file.
The way dynamips work prevent the update of the modification time.
We can improve that by using native system for watching file but:
* it's require dependencies specific for each OS
* dependencies use C extensions
* this is only a backup if your router is cleanly shutdown we export
stuff
