After maintaining previous behaviour in writing out the core notes, it's
time now to break with the past: do not write the PID in the first note.
Rationale:

1. [impact of the breakage] Process IDs in core files serve no immediate purpose to the debugger itself. They are only useful to relate a core file to a process. This can provide context to the person looking at the core file, provided one keeps track of this. Overall, not having the PID in the core file is only in very rare occasions unfortunate.
2. [reason of the breakage] Having one PRSTATUS note contain the PID, while all others contain the LWPID of the corresponding kernel thread creates an irregularity for the debugger that cannot easily be worked around. This is caused by libthread_db correlating user thread IDs to kernel thread (aka LWP) IDs and thus aware of the actual LWPIDs.

Update comments accordingly.