util: split all hostname related calls into hostname-util.c

src/network/networkd-dhcp4.c

@@ -22,6 +22,7 @@
 #include <netinet/ether.h>
 #include <linux/if.h>
 
+#include "hostname-util.h"
 #include "networkd-link.h"
 #include "network-internal.h"
 #include "dhcp-lease-internal.h"

src/network/networkd-network.c

@@ -25,6 +25,7 @@
 #include "conf-files.h"
 #include "conf-parser.h"
 #include "util.h"
+#include "hostname-util.h"
 #include "networkd.h"
 #include "networkd-netdev.h"
 #include "networkd-link.h"