chacha20_*_ctr functions now return the new ctr

chacha20_*_ctr functions now return the new ctr

This should facilitate building piecemeal streams.  Normally you'd just
increment the nonce, but in some (admittedly rare) cases we may want to
increment the counter instead.

Incrementing the counter is fairly dangerous, because we may overlap the
streams, thus revealing the XOR of two pieces of plain text. Using the
new return value makes sure this doesn't happen.