curl: add --parallel-max-host to limit concurrent connections per host

Where 'host' is protocol + hostname + portnumber.

Closes #18052

docs/cmdline-opts/Makefile.inc

@@ -184,6 +184,7 @@ DPAGES = \
   out-null.md \
   output.md \
   parallel-immediate.md \
+  parallel-max-host.md \
   parallel-max.md \
   parallel.md \
   pass.md \

docs/cmdline-opts/parallel-max-host.md

@@ -0,0 +1,28 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: parallel-max-host
+Arg: <num>
+Help: Maximum connections to a single host
+Added: 8.16.0
+Category: connection curl global
+Multi: single
+Scope: global
+See-also:
+  - parallel
+  - parallel-max
+Example:
+  - --parallel-max-host 5 -Z $URL ftp://example.com/
+---
+
+# `--parallel-max-host`
+
+When asked to do parallel transfers, using --parallel, this option controls
+the maximum amount of concurrent connections curl is allowed to do to the same
+protocol + hostname + port number target.
+
+The limit is enforced by libcurl and queued "internally", which means that
+transfers that are waiting for an available connection still look like started
+transfers in the progress meter.
+
+The default is 0 (unlimited). 65535 is the largest supported value.

docs/cmdline-opts/parallel-max.md

@@ -10,6 +10,7 @@ Multi: single
 Scope: global
 See-also:
   - parallel
+  - parallel-max-host
 Example:
   - --parallel-max 100 -Z $URL ftp://example.com/
 ---

docs/options-in-versions

@@ -151,6 +151,7 @@
 --parallel (-Z)                      7.66.0
 --parallel-immediate                 7.68.0
 --parallel-max                       7.66.0
+--parallel-max-host                  8.16.0
 --pass                               7.9.3
 --path-as-is                         7.42.0
 --pinnedpubkey                       7.39.0