We have added "dns" parameter to backend option.  If specified, name
lookup is done dynamically.  If not, name lookup is done at start up,
or configuration reloading.  nghttpx caches DNS result including error
case in 30 seconds in this commit.  Later commit makes this
configurable.

DNS resolution is done asynchronously using c-ares library.

Accept content-length: 0 in 204 response

Normally, we don't have wait for child process to exit, since init can
take care of them.  But in containerized environment, pid 0 init might
not be available, and defunct processes can be piled up.  This commit
ensures that OCSP and neverbleed processes are waited for before
worker process exits.

Strict http framing

We now treat Content-Length or Transfer-Encoding as error if they come
with 204 or 1xx status code, or 200 to a CONNECT request in HTTP/1
response.