zrtp: persistent ZID (thanks Ingo Feinerer)

the patch was contributed by Ingo Feinerer, thanks!

- add support for generating a random ZID the first time
  the application is started, and saving the ZID in
  $HOME/.baresip/zrtp_zid

- subsequent runs will read the ZID from this file

- setting client_id to "baresip/zrtp"

- setting license-mode to UNLIMITED