DHCPClient: Set Host Name option

This makes bare metal serenity systems reachable by their hostname
on my local network, which uses a FRITZ!Box as the router, DNS server,
and DHCP server. It appears to use this DHCP option to add DNS records
for "$hostname" and "$hostname.fritz.box".

Userland/Services/DHCPClient/DHCPv4Client.cpp

@@ -15,6 +15,7 @@
 #include <AK/ScopeGuard.h>
 #include <AK/Try.h>
 #include <LibCore/File.h>
+#include <LibCore/System.h>
 #include <LibCore/Timer.h>
 #include <stdio.h>
 
@@ -388,6 +389,10 @@ void DHCPv4Client::dhcp_request(DHCPv4Transaction& transaction, DHCPv4Packet con
     if (maybe_dhcp_server_ip.has_value())
         builder.add_option(DHCPOption::ServerIdentifier, sizeof(IPv4Address), &maybe_dhcp_server_ip.value());
 
+    auto hostname = Core::System::gethostname();
+    if (!hostname.is_error())
+        builder.add_option(DHCPOption::HostName, hostname.value().length(), hostname.value().characters());
+
     AK::Array<DHCPOption, 2> parameter_request_list = {
         DHCPOption::SubnetMask,
         DHCPOption::Router,