MFC 328134: Update various statements in vmstat(8) to match reality.

- The process stats are actually thread counts rather than process
counts.
- Simplify various descriptions to remove mention of stats that are
updated every 5 seconds (all VM related stats are now "instant",
only the load average is updated every 5 seconds).
- Don't make any mention of special treatment for processes that have
been active in the last 20 seconds. We don't track that stat.
- Rework the description of active virtual memory. Call it mapped
virtual memory and explicitly point out it is not the same as the
active page queue (which corresponds to "Active" in top(1)), and
also hint at the possible bogusness of the value (e.g. if a process
maps a single page out of a multiple GB file, the entire file's size
is considered mapped).
- Simplify a few descriptions that implied their output was a value
per interval. All of the "rate" values are per-second rates scaled
across the interval.
- Update a few comments for 'struct vmtotal' along similar lines.

MFC 283624,283630:
Export a list of VM objects in the system via a sysctl. The list can be
examined via 'vmstat -o'. It can be used to determine which files are
using physical pages of memory and how much each is using.

MFC 269727:
Update vmstat usage for last-argument count/wait parameters

Correct the usage in both the manpage and in usage() to indicate
that the wait interval and repetition count may be given either
with the respective -w/-c arguments, or as the final positional
arguments. [0]

The corresponding code to implement the positional arguments has
been conditional on the (always-enabled) BACKWARD_COMPATIBILITY
macro since the original 4.4-lite import. It's no longer reasonable
to remove the functionality, so remove the macro and conditional
instead.

Note that multiple disks may be given on the command line.

While here, sort arguments and apply minor mdoc fixes.

PR: 184755 [0]

MFC 283624,283630:
Export a list of VM objects in the system via a sysctl. The list can be
examined via 'vmstat -o'. It can be used to determine which files are
using physical pages of memory and how much each is using.

MFC 269727:
Update vmstat usage for last-argument count/wait parameters

Correct the usage in both the manpage and in usage() to indicate
that the wait interval and repetition count may be given either
with the respective -w/-c arguments, or as the final positional
arguments. [0]

The corresponding code to implement the positional arguments has
been conditional on the (always-enabled) BACKWARD_COMPATIBILITY
macro since the original 4.4-lite import. It's no longer reasonable
to remove the functionality, so remove the macro and conditional
instead.

Note that multiple disks may be given on the command line.

While here, sort arguments and apply minor mdoc fixes.

PR: 184755 [0]