quiche now requires the application to explicitly set the keylog path
for each connection, rather than reading the environment variable
itself.

Closes #5541

Syncs with ngtcp2 commit 7e9a917d386d98 merged June 7 2020.

Assisted-by: Tatsuhiro Tsujikawa
Closes #5538

Fix the verbose message while at it, quiche currently supports draft
27 and draft 28 simultaneously.

Closes #5518

When the method is updated inside libcurl we must still not change the
method as set by the user as then repeated transfers with that same
handle might not execute the same operation anymore!

This fixes the libcurl part of #5462

Test 1633 added to verify.

Closes #5499

Tested with ngtcp2 built against the OpenSSL library. Additionally
tested with MultiSSL (NSS for TLS and ngtcp2+OpenSSL for QUIC).

The TLS backend (independent of QUIC) may or may not already have opened
the keylog file before. Therefore Curl_tls_keylog_open is always called
to ensure the file is open.

Reported-by: Peter Wu
Fixes #5447 (the ngtcp2 side of it)
Closes #5451

Addresses the quiche side of #5447
Reported-by: Peter Wu
Closes #5450

Based on client.cc changes from ngtcp2. Tested with current git master,
ngtcp2 commit c77d5731ce92, nghttp3 commit 65ff479d4380.

Fixes #5444
Closes #5443

If the QLOGDIR environment variable is set, enable qlogging.

... and create Curl_qlogdir() in the new generic vquic/vquic.c file for
QUIC functions that are backend independent.

Closes #5353

quiche has the potential to log qlog files. To enable this, you must
build quiche with the qlog feature enabled `cargo build --features
qlog`. curl then passes a file descriptor to quiche, which takes
ownership of the file. The FD transfer only works on UNIX.

The convention is to enable logging when the QLOGDIR environment is
set. This should be a path to a folder where files are written with the
naming template <SCID>.qlog.

Co-authored-by: Lucas Pardue
Replaces #5337
Closes #5341