units(1): Add v option: verbose

For increased compatibility with GNU units: support a -v option which
produces more verbose output when spitting out the answer.
GNU -v does additional work in the version, information, and check output which
we do not (yet?) replicate.
Refine r264422: set buf to NULL only when we don't allocate memory,
and free buf unconditionally.

Requested by:	kib
MFC after:	1 week
units(1): Add 'static' and sort list.

Inspired by:	DragonflyBSD
units(1): don't compare pointers with 0

For better readability, don't compare pointers with 0 or assign 0
to them. Use NULL instead.

Inspired by:	DragonflyBSD