there was a function to "abbreviate" sizes, but it was somewhat
unclear and incomplete. reuse the sizeof_fmt_* set of functions from
the borg backup project (MIT licensed) to implement a more complete
and flexible display that will scale up to the Yottabyte and
beyond. it also supports non-IEC units (like "kibibyte", AKA 1024
bytes) if you fancy that stuff.
this is a workaround for #91: it allows users to better see the size
of the file that will be transfered.
*some* places are still kept in bytes, most notably when receive fails
to receive all bytes ("got %d bytes, wanted %d") because we may want
more clarity there.
text transfers also use the "bytes" suffix (instead of "B") because it
will commonly not reach beyond the KiB range.
note that the test suite only covers decimal (non-IEC) prefix, but it
is assumed to be sufficient to be considered correct.
