unix(4): Enhance LOCAL_CREDS_PERSISTENT ABI

As this ABI is still fresh (r367287), let's correct some mistakes now:

• Version the structure to allow for future changes
• Include sender's pid in control message structure
• Use a distinct control message type from the cmsgcred / sockcred mess

Discussed with: kib, markj, trasz
Differential Revision: https://reviews.freebsd.org/D27084