Implement faster formatting for format_iso8601 and format_common_log
nghttpx: Note the conversion made in <VAR>
nghttpx: Make --accesslog-format usage doc help2man friendly

This is unfortunate but help2man behaves badly when there is indented
lines in help messages in commnad-line args.  We removed indentations
to make help2man happy.
nghttpx: Add $pid to --accesslog-format variable

$pid refers to the PID of the running process.
nghttpx: Add $alpn variable to accesslog formatting

$alpn is a variable which represents ALPN identifier of the protocol
which generates the response.