curl: add --proxy-http2

For trying HTTP/2 with an HTTPS proxy.

Closes #10926

docs/cmdline-opts/Makefile.inc

@@ -178,6 +178,7 @@ DPAGES = \
   proxy-crlfile.d \
   proxy-digest.d \
   proxy-header.d \
+  proxy-http2.d \
   proxy-insecure.d \
   proxy-key-type.d \
   proxy-key.d \

docs/cmdline-opts/proxy-http2.d

@@ -0,0 +1,18 @@
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: proxy-http2
+Tags: Versions HTTP/2
+Protocols: HTTP
+Added: 8.1.0
+Mutexed:
+Requires: HTTP/2
+See-also: proxy
+Help: Use HTTP/2 with HTTPS proxy
+Category: http proxy
+Example: --proxy-http2 -x proxy $URL
+Multi: boolean
+---
+Tells curl to try negotiate HTTP version 2 with a HTTPS proxy. The proxy might
+still only offer HTTP/1 and then curl will stick to using that version.
+
+This has no effect for any other kinds of proxies.

docs/options-in-versions

@@ -165,6 +165,7 @@
 --proxy-crlfile                      7.52.0
 --proxy-digest                       7.12.0
 --proxy-header                       7.37.0
+--proxy-http2                        8.1.0
 --proxy-insecure                     7.52.0
 --proxy-key                          7.52.0
 --proxy-key-type                     7.52.0