feat: dhcpv4: send current hostname, fix spec compliance of renewals

This adds support for automatically registering node hostnames in DNS by
sending the current hostname to DHCP via option 12. If the current hostname is
updated, issue a new DISCOVER to propagate the update to DHCP (updating the
hostname on lease renewals is not universally supported by DHCP servers). This
addition maintains the previous functionality where the node can also request
its hostname from the DHCP server. The received hostname will be processed and
prioritized as usual by the `network.HostnameSpecController`.

This change set also contains fixes to make DHCP renewals compliant with RFC
2131, specifically avoiding sending the server identifier and requested IP
address when issuing renewals using a previous offer. This also uncovered an
issue in the upstream `insomniacslk/dhcp` library, for which a fix is now
pending at insomniacslk/dhcp#469. As upstream
contributions seem to have stalled, I have temporarily added a replacement for
the library to my own fork that carries the fix.

Sending hostname updates have been tested against `dnsmasq` and the built-in
DHCP + DNS services in Windows Server. Hostname retrieval from DHCP and edge
cases with overridden hostnames from different configuration layers have been
extensively tested against `dnsmasq`.

Signed-off-by: Dennis Marttinen <[email protected]>

go.mod

@@ -3,6 +3,10 @@ module github.com/siderolabs/talos
 go 1.20
 
 replace (
+	// Forked insomniacslk/dhcp with DHCP spec compliancy fixes
+	// Upstream PR: https://github.com/insomniacslk/dhcp/pull/469
+	github.com/insomniacslk/dhcp => github.com/twelho/dhcp v0.0.0-20220911105712-2eae1fc34ef5
+
 	// Use nested module.
 	github.com/siderolabs/talos/pkg/machinery => ./pkg/machinery