libc: tests: add some tests for cpuset(2)

The cpuset(2) tests should be run as root (require.user properly set) with

3 cpus for maximum coverage. All tests that want to modify the cpuset

don't assume any particular cpu layout (i.e. the first cpu may not be 0, the
last may not be first + count) and the following scenarios are tested:

1.) newset: basic execute cpuset() to grab a new cpuset, make sure the

assigned cpuset then has a different ID.

2.) transient: create a new cpuset then assign the process its original

cpuset, ensuring that the one we created is now gone.

3.) deadlk: test assigning an anonymous mask, then resetting the process

base affinity with 1-cpu overlap w.r.t. the anonymous mask and with
0-cpu overlap w.r.t. the anonymous mask.

4.) jail_attach_newbase: process attaches to a jail with its own

cpuset+mask (e.g. cpuset -c -l 1,2 jail -c path=/ command=/bin/sh)

5.) jail_attach_newbase_plain: process attaches to a jail with its own

cpuset (e.g. cpuset -c jail -c path=/ command=/bin/sh)

6.) jail_attach_prevbase: process attaches to a jail with the containing

jail's root cpuset (e.g. jail -c path=/ command=/bin/sh)

7.) jail_attach_plain: process attaches to a jail with the containing jail's

root cpuset+mask.

8.) badparent: creates a new cpuset and modifies the anonymous thread mask,

then setid's back to the original and checks that cpuset_getid() returns
the expected set.

Differential Revision: https://reviews.freebsd.org/D27307