output/srzip: rename variable for archive unit size

output/srzip: rename variable for archive unit size

The srzip output module needs to deal with two unit sizes. One that got
derived from the device's or input's logic channels' total count, and
will end up being used in the ZIP archive. And one which is seen when
the session feed provides sample data.

Rename the identifier to reflect this distinction. See a word diff for
the essence of the change.

Also rephrase the zip_append_queue() invocation for improved
readability. Use line breaks to illustrate parameter sets which are
related, and separate them from other parameters which are unrelated.