ptrace: do not allow for parallel ptrace requests
Set a new P2_PTRACEREQ flag around the request Wait for the target .
process P2_PTRACEREQ flag to clear before setting ours .
Otherwise, we rely on the moment that the process lock is not dropped
until the stopped target state is important. This is going to be no
longer true after some future change.
Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D29955