So that you can specify --read-rate=1M --read-burst=4M

When same SSL_CTX is used by multiple thread simultaneously we have to
setup some number of mutex locks for it.  We could not check how this
locking affects scalability since we have 4 cores at best in our
development machine.  Good side of sharing SSL_CTX across threads is
we can share session ID pool.

If --tls-ctx-per-worker is enabled, SSL_CTX is created per thread
basis and we can eliminate mutex locks.  The downside is session ID is
no longer shared, which means if session ID generated by one thread
cannot be acceptable by another thread.  But we have now session
ticket enabled and its keys are shared by all threads.

With androideabi-4.9, android build now supports threading.

This also fixes the bug that nghttpx's acceptor fd is blocking if
SOCK_NONBLOCK is undefined.

Thanks to Pascal

Connection failure is handled by connected() and other errors lead to
just failure, no retry is needed.

Previously to create manual page for bundled programs, we use help2man
to create man page from program's help output.  Then our man2rst.py
script converts man page to rst document.  Sphinx generates html from
rst documents.

Now help2rst.py produces rst document from programs output.  We use
Sphinx solely to produce both man pages and html files.

This reverts commit 742d80aa.

SSL_write requires the same arguments (buf pointer and its length) on
SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE.  get_write_limit() may
return smaller length than previously passed to SSL_write, which
violates OpenSSL assumption.  To avoid this, we keep last legnth
passed to SSL_write to tls_last_writelen_ if SSL_write indicated I/O
blocking.