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]
Approved by:	hrs (mentor, src committer)