curl: support VLAN Priority: --vlan-priority

Add --vlan-priority option to the command line tool for setting VLAN
priority.

Closes #13907

docs/cmdline-opts/Makefile.inc

@@ -304,5 +304,6 @@ DPAGES = \
   variable.md \
   verbose.md \
   version.md \
+  vlan-priority.md \
   write-out.md \
   xattr.md

docs/cmdline-opts/ip-tos.md

@@ -10,6 +10,7 @@ Protocols: All
 Multi: single
 See-also:
   - tcp-nodelay
+  - vlan-priority
 Example:
   - --ip-tos CS5 $URL
 ---

docs/cmdline-opts/vlan-priority.md

@@ -0,0 +1,23 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: vlan-priority
+Arg: <priority>
+Help: Set VLAN priority
+Added: 8.9.0
+Category: connection
+Protocols: All
+Multi: single
+See-also:
+  - ip-tos
+Example:
+  - --vlan-priority 4 $URL
+---
+
+# `--vlan-priority`
+
+Set VLAN priority as defined in IEEE 802.1Q. (Added in 8.9.0).
+
+This field is set on Ethernet level, and only works within a local network.
+
+The valid range for \<priority\> is 0 to 7.

docs/options-in-versions

@@ -269,5 +269,6 @@
 --variable                           8.3.0
 --verbose (-v)                       4.0
 --version (-V)                       4.0
+--vlan-priority                      8.9.0
 --write-out (-w)                     6.5
 --xattr                              7.21.3