lib: Move print_rate() from tc here; modernize

lib: Move print_rate() from tc here; modernize

The functions print_rate() and sprint_rate() are useful for formatting
rate-like values. The DCB tool would find these useful in the maxrate
subtool. However, the current interface to these functions uses a global
variable use_iec as a flag indicating whether 1024- or 1000-based powers
should be used when formatting the rate value. For general use, a global
variable is not a great way of passing arguments to a function. Besides, it
is unlike most other printing functions in that it deals in buffers and
ignores JSON.

Therefore make the interface to print_rate() explicit by converting use_iec
to an ordinary parameter. Since the interface changes anyway, convert it to
follow the pattern of other json_print functions (except for the
now-explicit use_iec parameter). Move to json_print.c.

Add a wrapper to tc, so that all the call sites do not need to repeat the
use_iec global variable argument, and convert all call sites.

In q_cake.c, the conversion is not straightforward due to usage of a macro
that is shared across numerous data types. Simply hand-roll the
corresponding code, which seems better than making an extra helper for one
call site.

Drop sprint_rate() now that everybody just uses print_rate().

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@gmail.com>