Previously, there is inconsistency when on_frame_recv_callback
is called between HEADERS/PUSH_PROMISE and the other frames.
For former case, it is called before header block, in latter
case, it is called after whole frame is received. To make it
consistent, we call on_frame_recv_callback for HEADERS/PUSH_PROMISE
after its frame is fully received. Since on_frame_recv_callback
can signal the end of header block, we replaced on_end_headers_callback
with on_begin_headers_callback, which is called when the reception
of the header block is started.
The option syntax is <PATH>=<PUSH_PATH,...>.
Push resources PUSH_PATHs when PATH is requested. This option can be used
repeatedly to specify multiple push configurations. For example,
-p/=/foo.png -p/doc=/bar.css
PATH and PUSH_PATHs are relative to document root.