MFC r318325:

Start writing up some basic feature tests for procstat

These tests query a running process for information related to the -b,
-c, -e, and -f flags; the -f testcase is largely stubbed out, pending
additional work to determine a good, deterministic descriptor.

Core file test support is coming soon--it requires a bit more effort
due to the fact that:
- coredumps can be disabled (kern.coredump=0).
- corefiles can be put in different directories than the current
directory, or be named something other than `<prog>.core`
(`kern.corefile`).

MFC r279842, r279875

Implement basic reporting of cpuset info via the -S option

Obtained from: Netflix, Inc.

MFC 266293:
- Add support for dumping current resource usage for processes via a new -r
flag to procstat.
- Add an -H flag to request information about threads rather than processes
when dumping statistics. Currently it is only used for -r to display
resource usage for individual threads instead of the entire process.

MFC r279842, r279875

Implement basic reporting of cpuset info via the -S option

Obtained from: Netflix, Inc.

MFC 266293:
- Add support for dumping current resource usage for processes via a new -r
flag to procstat.
- Add an -H flag to request information about threads rather than processes
when dumping statistics. Currently it is only used for -r to display
resource usage for individual threads instead of the entire process.