libwebsockets

h2: unify immortal stream tracking across SSE and ws substreams

It was already correct but add helpers to isolate and deduplicate
processing adding and closing a generically immortal stream.

Change the default 31s h2 network connection timeout to be settable
by .keepalive_timeout if nonzero.

Add a public api allowing a client h2 stream to transition to
half-closed LOCAL (by sending a 0-byte DATA with END_STREAM) and
mark itself as immortal to create a read-only long-poll stream
if the server allows it.

Add a vhost server option flag LWS_SERVER_OPTION_VH_H2_HALF_CLOSED_LONG_POLL
which allows the vhost to treat half-closed remotes as immortal long
poll streams.