When parsing the -b (bandwidth) argument, use 10-based suffices.

We support using k, m, and g as suffices on input values.  In most
cases these are 2-based suffixes (i.e. K == 1024) because they are
sizes of objects.  In the case of rates, we need to use 10-based
suffices (i.e. K == 1000).

We do this by implementing (using copy-and-paste) a unit_atof_rate()
subroutine that parses strings similarly to unit_atof but using
10-based suffices instead.

Fixes #173.