chacha: add function to set initial block counter

The ChaCha20 based header protection algorithm in QUIC requires a way
to set the initial value of counter:
https://quicwg.org/base-drafts/draft-ietf-quic-tls.html#name-chacha20-based-header-prote



This will add a new function chacha_set_counter, which takes an
8-octet initial value of the block counter.

Signed-off-by: Daiki Ueno <dueno@redhat.com>