Move hostname setup logic to new shared/hostname-setup.[ch]

No functional change, just moving a bunch of things around. Before
we needed a rather complicated setup to test hostname_setup(), because
the code was in src/core/. When things are moved to src/shared/
we can just test it as any function.

The test is still "unsafe" because hostname_setup() may modify the
hostname.

src/nspawn/nspawn.c

@@ -46,6 +46,7 @@
 #include "fs-util.h"
 #include "gpt.h"
 #include "hexdecoct.h"
+#include "hostname-setup.h"
 #include "hostname-util.h"
 #include "id128-util.h"
 #include "io-util.h"