PYRO PSK: adjusted cipher and key length to be usable with openSSL 1.0.2k.

Automatically get PSK and ID when connecting to PYRO[S], so that future connection through PYROS can use that same key. Also fixed case to UPPER for *PSK.

Runtime : Ensure that a random PSK secret compatible with stunnel is generated if -s commandline switch is used. Stunnel service is restarted after generation, using spawn_subprocess. TODO : give stunnel restart command as a commandline parameter.

Added runtime/spawn_subprocess.py. Force use posix spawn instead of fork, with API similar to subprocess. Using fork in runtime is incompatible with Xenomai, because memory is locked and this can lead to out of memory error.