Set TDP_KTHREAD before calling cpu_fork() and cpu_copy_thread().

[jhb]

Summary

This permits these routines to use special logic for initializing MD
kthread state.

For the kproc case, this required moving the logic to set these flags
from kproc_create() into do_fork().

[kib]

sys/kern/kern_fork.c

509

Note that code for DROPSIG_CAUGHT above is accurate to lock p2 ps_mtx around p_sigacts manipulations (even if only for asserts). I think it is better to be consistent both by style and formal requirements and lock ps_mtx if (FR2_DROPSIG_CAUGHT | FR2_KPROC).

[jhb]

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