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/fuzz/fuzz-hostname-setup.c

@@ -4,7 +4,7 @@
 #include "fd-util.h"
 #include "fileio.h"
 #include "fuzz.h"
-#include "hostname-util.h"
+#include "hostname-setup.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_fclose_ FILE *f = NULL;

src/fuzz/meson.build

@@ -123,7 +123,7 @@ fuzzers += [
          [libshared],
          []],
 
-        [['src/fuzz/fuzz-hostname-util.c'],
+        [['src/fuzz/fuzz-hostname-setup.c'],
          [libshared],
          []],