As part of normal HTTP/2 processing, incomplete frames are saved in the
control state using a fixed size memcpy of NGX_HTTP_V2_STATE_BUFFER_SIZE.
For this matter, two state buffers are reserved in the HTTP/2 recv buffer.

As part of HTTP/2 auto-detection on plain TCP connections, initial data
is first read into a buffer specified by the client_header_buffer_size
directive that doesn't have state reservation.  Previously, this made it
possible to over-read the buffer as part of saving the state.

The fix is to read the available buffer size rather than a fixed size.
Although memcpy of a fixed size can produce a better optimized code,
handling of incomplete frames isn't a common execution path, so it was
sacrificed for the sake of simplicity of the fix.

It is made local as it is only needed now when creating crypto context.

BoringSSL lacks EVP interface for ChaCha20, providing instead
a function for one-shot encryption, thus hp is still preserved.

Based on a patch by Roman Arutyunyan.

After conversion to reusable crypto ctx, now there's enough caller
context to remove the "level" argument from ngx_quic_ciphers().