dhcp6: implement vendor class support

Implement the OT_DHCP6_VENDOR option type to properly handle the
DHCPv6 Vendor Class option (code 16) according to RFC 3315.

Previously, this option was marked as OT_INTERNAL, causing dnsmasq
to fail immediately if configured by name. Bypassing this block by
using the numerical option format (option6:16) caused the payload
to be formatted with an unintended string-length prefix. This broke
compliance because the RFC requires a fixed 4-byte Enterprise ID
at the beginning of the option, followed by data blocks.

This byte misalignment broke features like UEFI HTTP IPv6 Boot

This patch removes the OT_INTERNAL restriction and moves the formatting
logic to the parser phase, correctly assembling the wire-format layout
(4-byte ID + 2-byte chunk length + string) so it can be injected
directly into the network buffer.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>

man/dnsmasq.8

@@ -1372,11 +1372,13 @@ described in RFC 3442.
 IPv6 options are specified using the \fBoption6:\fP
 keyword, followed by the option number or option name. The IPv6 option
 name space is disjoint from the IPv4 option name space. IPv6 addresses
-in options must be bracketed with square brackets, eg. 
-\fB --dhcp-option=option6:ntp-server,[1234::56]\fP. 
+in options must be bracketed with square brackets, eg. \fB --dhcp-option=option6:ntp-server,[1234::56]\fP. 
 For IPv6, [::] means "the global address of
 the machine running dnsmasq", whilst [fd00::] is replaced with the
-ULA, if it exists, and [fe80::] with the link-local address.
+ULA, if it exists, and [fe80::] with the link-local address. The vendorclass option
+is special for DHCPv6, since it requires an enterprise number which must appear as a decimal number
+before the class(es). eg. \fB --dhcp-option=option6:vendor-class,343,HTTPClient\fP
+
 
 Be careful: data-type suitability for the option number sent is not checked.
 It is quite possible to persuade dnsmasq to generate illegal DHCP packets with