vquic: ngtcp2 + openssl support

With the new addition of OpenSSL QUIC API support and the support in
ngtcp2 main branch, make the necessary adjustments in curl to support
this combination.

- add support in configure.ac to detect the feature OPENSSL_QUIC_API2 in
  openssl
- initialise ngtcp2 properly in this combination
- add a Curl_vquic_init() for global initialisation that ngtcp2 likes
  for performance reasons
- add documentation on how to build in docs/HTTP3.md
- add CI testing in http3-linux.yml

Assisted-by: Viktor Szakats
Closes #17027

docs/HTTP3.md

@@ -50,6 +50,50 @@ improvements.
 The build examples use `$NGHTTP3_VERSION` and `$NGTCP2_VERSION` as placeholders
 for the version you build.
 
+## Build with OpenSSL
+
+OpenSSL v3.5.0+ offers APIs for integration with *ngtcp2* v1.12.0+. Earlier
+versions do not work.
+
+Build OpenSSL (version 3.5.0 or newer):
+
+     % git clone --quiet --depth=1 -b openssl-$OPENSSL_VERSION https://github.com/openssl/openssl
+     % cd openssl
+     % ./config --prefix=<somewhere1> --libdir=lib
+     % make
+     % make install
+
+Build nghttp3:
+
+     % cd ..
+     % git clone -b $NGHTTP3_VERSION https://github.com/ngtcp2/nghttp3
+     % cd nghttp3
+     % git submodule update --init
+     % autoreconf -fi
+     % ./configure --prefix=<somewhere2> --enable-lib-only
+     % make
+     % make install
+
+Build ngtcp2:
+
+     % cd ..
+     % git clone -b $NGTCP2_VERSION https://github.com/ngtcp2/ngtcp2
+     % cd ngtcp2
+     % autoreconf -fi
+     % ./configure PKG_CONFIG_PATH=<somewhere1>/lib/pkgconfig:<somewhere2>/lib/pkgconfig LDFLAGS="-Wl,-rpath,<somewhere1>/lib" --prefix=<somewhere3> --enable-lib-only --with-openssl
+     % make
+     % make install
+
+Build curl:
+
+     % cd ..
+     % git clone https://github.com/curl/curl
+     % cd curl
+     % autoreconf -fi
+     % LDFLAGS="-Wl,-rpath,<somewhere1>/lib" ./configure --with-openssl=<somewhere1> --with-nghttp3=<somewhere2> --with-ngtcp2=<somewhere3>
+     % make
+     % make install
+
 ## Build with quictls
 
 OpenSSL does not offer the required APIs for building a QUIC client. You need
@@ -59,7 +103,7 @@ Build quictls (any `+quic` tagged version works):
 
      % git clone --depth 1 -b openssl-3.1.4+quic https://github.com/quictls/openssl
      % cd openssl
-     % ./config enable-tls1_3 --prefix=<somewhere1>
+     % ./config enable-tls1_3 --prefix=<somewhere1> --libdir=lib
      % make
      % make install
 
@@ -94,8 +138,6 @@ Build curl:
      % make
      % make install
 
-For OpenSSL 3.0.0 or later builds on Linux for x86_64 architecture, substitute all occurrences of "/lib" with "/lib64"
-
 ## Build with GnuTLS
 
 Build GnuTLS: