With 4825009e, static lib is no longer
required for unit tests.

This function is useful for the client application to know that there
is a chance that request can be sent.  If this function returns 0,
there is zero chance to make a request.

This commit also set error_code passed to
nghttp2_on_stream_close_callback to NGHTTP2_REFUSED_STREAM if request
is not sent.

If request submission is failed, make all remaining requests for that
client fail.

User can change file name using --mime-types-file option.

It seems that using ev_feed_event to signal write operation is much
faster than starting watcher.  This is probably due to the fact that
we don't need to wait in event loop.  The same thing cannot be done in
HTTP/2 frontend, since this will raise write operation for each stream
HEADER/DATA write, which leads to very small packets, hurting
performance.  Interestingly, HTTP/1 frontend also suffers the same
performance hit.

Revert "add PIDFile directive to systemd service"

h2load: do not overwrite protocol value

This reverts commit 19c095a8.

See discussion at #375 for details.

This commit also fixes the problem that application protocol is not
shown if cleartext spdy is used.

Update doc for Debian 7.0 or above and add g++ requirement