units(1): make -V print version instead of -v

The units program is likely little used.  It is even less likely that a script
will want the units program to print out its version number by passing -v.

GNU units uses -V for version and -v for verbosity.

Increase compatibility between these two versions (written by the same author)
by switching our flag as well.

Take this opportunity to remove bogus information about the version number and
just call it 'FreeBSD units'.

Discussed with:	cperciva, rwatson