syscalls: if ptrace(PT_SETREG) occured during syscall, do not do set_syscall_retval()

[kib]

Summary

to preserve the register state enforced by debugger.  This is only
relevant for transparent attach to the slow syscalls where sleeps are
not interrupted.

PR:     290008


kern/subr_syscalls.c: declare 'traced' local variables boolean

[kib]

That said, I disagree that this is a kernel bug at all.

[markj]

sys/kern/subr_syscall.c

119

Don't we need the proc lock here?

[kib]

Clear TDB_USERWR under proc lock. Do it earlier.

[markj]

I'm not sure if we should make such a change, but it looks ok to me.

[kib]

Rebase