Fixes GH-207

We should ignore Content-Length and Transfer-Encoding for upgraded
response, and reset content-length if this is a non-final response.

Although posx_spawn is very convenient and useful, we have platform
which don't have these functions (e.g., Android NDK r10d).

Since libev handles SIGCHLD, using waitpid in separate thread to wait
for the completion of fetch-ocsp-response script process is undefined.
This commit rewrite ocsp handling code so that it utilizes libev
ev_child watcher and perform ocsp update without thread.

Previously we send RST_STREAM when we send DATA with END_STREAM flag
set.  With this commit, we also do this when we send HEADERS with
END_STREAM flag set.

The bug was introduced by 8c3b379b.