racct: Fix accounting of CPU time for the system idle process

[markj]

Summary

• Add a flag which cleanly indicates that a given process is the system idle process.
• Modify racctd() to skip over the idle proc when aggregating CPU time and other metrics which don't apply to the idle thread.
• Remove handling for idle threads from racct_getpcpu().

PR: 269097
MFC after: 2 weeks
Sponsored by: Klara, Inc.

[kib]

sys/kern/kern_racct.c

314

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