fork: avoid endless wait with PTRACE_FORK and RFSTOPPED.

An RFSTOPPED thread can't clean TDB_STOPATFORK, which is done in the
fork_return() in its context, so parent is stuck forever. Triggered
when trying to ptrace linux process. Instead of waiting for the new
thread to clear TDB_STOPATFORK, tag it as traced and reparent to the
debugger in do_fork(), and let it only notify the debugger when run.

Submitted by: Yanko Yankulov <yanko.yankulov@gmail.com>
Reviewed by: jhb
MFC after: 1 week
X-MFC-Note: keep p_dbgwait placeholder intact
Differential revision: https://reviews.freebsd.org/D15857