bug#70645: Reliable HTTPS networking

For years now I've been trying to work out how to do reliable HTTPS
networking with Guile, where reliable just means that it can't hang
indefinitely.

After a few wrong turns, I believe the way to do this is use
non-blocking ports as that combined with suspendable ports in Guile
allows you to provide current-read-waiter/current-write-waiter
procedures that will timeout at some point.

I think the final hurdle is to get tls-wrap in (web client) to support
Asynchronous operation with GnuTLS [1] and I think there are only a
couple of things missing. make-session needs passing
connection-flag/nonblock and error/again plus error/interrupted
exceptions need handling for the handshake using the information from
record-get-direction about whether Guile should wait to write or read.

1: https://gnutls.org/manual/html_node/Asynchronous-operation.html

I think I forgot to move things forward after guile-gnutls 4.0.0
released with record-get-direction, so I'm opening this bug to try and
keep track of things.

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