tree-wide: Introduce sd-forward.h and shared-forward.h headers

Let's not leak details from src/shared and src/libsystemd into
src/basic, even though you can't actually do anything useful with
just forward declarations from src/shared.

The sd-forward.h header is put in src/libsystemd/sd-common as we
don't have a directory for shared internal headers for libsystemd
yet.

Let's also rename forward.h to basic-forward.h to keep things
self-explanatory.

src/basic/ether-addr-util.h

@@ -5,7 +5,7 @@
 #include <netinet/in.h>
 #include <net/ethernet.h>
 
-#include "forward.h"
+#include "basic-forward.h"
 
 /* This is MAX_ADDR_LEN as defined in linux/netdevice.h, but net/if_arp.h
  * defines a macro of the same name with a much lower size. */