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.

It seems that returning 0 when enc == 0 crashes OpenSSL.

This option specifies files contains 48 random bytes to construct
session ticket key data.  This option can be used repeatedly to
specify multiple keys, but only the first one is used to encrypt
tickets.

On reception of GOAWAY, new stream creation is disallowed regardless
of last-stream-id in GOAWAY is larger than next stream ID.

Update is done by main event loop which is stopped after graceful
shutdown is commenced, which means time is no longer update.  To avoid
this situation, we just avoid caching and get time for each logging.

This is because std::chrono::high_resolution_clock may not be wall
clock; it may be alias of std::chrono::stead_clock.