network: Implement DHCP Option 119 (Domain Search List) (#5932)

This adds a modified version of dhcp6_option_parse_domainname() that is
able to parse compressed domain names, borrowing the idea from
dns_packet_read_name(). It also adds pieces in networkd-link and
networkd-manager to properly save/load the added option field.

Resolves #2710.

src/test/meson.build

@@ -800,6 +800,12 @@ tests += [
           libsystemd_network],
          []],
 
+        [['src/libsystemd-network/test-sd-dhcp-lease.c',
+          'src/libsystemd-network/dhcp-lease-internal.h'],
+         [libshared,
+          libsystemd_network],
+         []],
+
         [['src/libsystemd-network/test-dhcp-client.c',
           'src/libsystemd-network/dhcp-protocol.h',
           'src/libsystemd-network/dhcp-internal.h',