Stricter checks for pseudo-headers :method and :path

Check the allowed characters for ":method" (see RFC 7230, section 3.2.6) and
":path". For ":path", the space and tab characters are now forbidden, but
other special characters are still allowed for compatibility reasons.

Update genvchartbl.py so that it generates the same table as in the code.

Fixes #1611

Forward QUIC UDP datagram to lingering worker process which is in
graceful shutdown.  Both SIGHUP and SIGUSR2 work.  To make this work
correctly, eBPF is required.