cpuset_setproc: use the appropriate parent for new anonymous sets

cpuset_setproc: use the appropriate parent for new anonymous sets

As far as I can tell, this has been the case since initially committed in
2008.  cpuset_setproc is the executor of cpuset reassignment; note this
excerpt from the description:

* 1) Set is non-null.  This reparents all anonymous sets to the provided
*    set and replaces all non-anonymous td_cpusets with the provided set.

However, reviewing cpuset_setproc_setthread() for some jail related work
unearthed the error: if tdset was not anonymous, we were replacing it with
`set`. If it was anonymous, then we'd rebase it onto `set` (i.e. copy the
thread's mask over and AND it with `set`) but give the new anonymous set
the original tdset as the parent (i.e. the base of the set we're supposed to
be leaving behind).

The primary visible consequences were that:

1.) cpuset_getid() following such assignment returns the wrong result, the
    setid that we left behind rather than the one we joined.
2.) When a process attached to the jail, the base set of any anonymous
    threads was a set outside of the jail.

This was initially bundled in D27298, but it's a minor fix that's fairly
easy to verify the correctness of.

A test is included in D27307 ("badparent"), which demonstrates the issue
with, effectively:

osetid = cpuset_getid()
newsetid = cpuset()
cpuset_setaffinity(thread)
cpuset_setid(osetid)
cpuset_getid(thread) -> observe that it matches newsetid instead of osetid.

MFC after:	1 week