Move gdhcp and miracle-dhcp to ./src/dhcp/

Use a common subdirectory for gdhcp so we can easily replace it once
sd-dhcp has server-side support.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

src/dhcp/dhcp.c

@@ -69,7 +69,7 @@
 #include <sys/socket.h>
 #include <sys/wait.h>
 #include <unistd.h>
-#include "gdhcp/gdhcp.h"
+#include "dhcp/gdhcp.h"
 #include "shl_log.h"
 
 static const char *arg_netdev;