procdesc: allow to collect status through wait(1) if process is traced

The debugger like truss(1) depends on the wait(2) syscall. This syscall
waits for ALL children. When it is waiting for ALL child's the children
created by process descriptors are not returned. This behavior was
introduced because we want to implement libraries which may pdfork(1).

The behavior of process descriptor brakes truss(1) because it will
not be able to collect the status of processes with process descriptors.

To address this problem the status is returned to parent when the
child is traced. While the process is traced the debugger is the new parent.
In case the original parent and debugger are the same process it means the
debugger explicitly used pdfork() to create the child. In that case the debugger
should be using kqueue()/pdwait() instead of wait().

Add test case to verify that. The test case was implemented by markj@.

Reviewed by: kib, markj
Discussed with: jhb
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D20362