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

• 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
Reviewed by: olce, kib
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50073