The HEADERS frame is always represented by more than one buffer since
b930e598a199, but the handling code hasn't been adjusted.

Only the first buffer of HEADERS frame was checked and if it had been
sent while others had not, the rest of the frame was dropped, resulting
in broken connection.

Before b930e598a199, the problem could only be seen in case of HEADERS
frame with CONTINUATION.

The r->invalid_header flag wasn't reset once an invalid header appeared in a
request, resulting in all subsequent headers in the request were also marked
as invalid.

The directive toggles conversion of HEAD to GET for cacheable proxy requests.
When disabled, $request_method must be added to cache key for consistency.
By default, HEAD is converted to GET as before.

OpenSSL doesn't check if the negotiated protocol has been announced.
As a result, the client might force using HTTP/2 even if it wasn't
enabled in configuration.

It caused inconsistency between setting "in_closed" flag and the moment when
the last DATA frame was actually read.  As a result, the body buffer might not
be initialized properly in ngx_http_v2_init_request_body(), which led to a
segmentation fault in ngx_http_v2_state_read_data().  Also it might cause
start processing of incomplete body.

This issue could be triggered when the processing of a request was delayed,
e.g. in the limit_req or auth_request modules.

Now it limits only the maximum length of literal string (either raw or
compressed) in HPACK request header fields.  It's easier to understand
and to describe in the documentation.

No functional changes.