... since gcc -pedantic warns on it.

Currently, we use same number of HTTP/2 sessions per worker with given
backend addresses.  New option to specify the number of HTTP/2 session
per worker will follow.

It seems that we don't care about this since we don't change buffer
pointer between would-block write/read and next write/read.  Somehow
we decided we need these fields.  As a precaution, we set
SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER in SSL_set_mode() for both server
and client contexts.

It turns out that writing successfully to network is not enough.
After apparently successful network write, read fails and then we
first know network has been lost (at least my android mobile network).
In this change, we say connection check is successful only when
successful read.  We already send PING in this case, so we just wait
PING ACK with short timeout.  If timeout has expired, drop connection.
Since waiting for PING ACK could degrade performance for fast reliably
connected network, we decided to disable connection check by default.
Use --backend-http2-connection-check to enable it.

Downstream's headers mutation functions have been rewritten to share
code.