tail: Fix -b, -c, -n options

Switching from strtoll() to expand_number() was improper at the time as
it only accepted positive numbers. Now that it also accepts negative
numbers, the -b option is still broken because the same commit that
switched to expand_number() also dropped the multiplication by units.

Fixes: 643ac419fafb
Reviewed by: delphij
Differential Revision: https://reviews.freebsd.org/D51757