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/basic/hostname-util.h

@@ -7,8 +7,6 @@
 #include "macro.h"
 #include "strv.h"
 
-bool hostname_is_set(void);
-
 char* gethostname_malloc(void);
 char* gethostname_short_malloc(void);
 int gethostname_strict(char **ret);
@@ -29,10 +27,3 @@ static inline bool is_gateway_hostname(const char *hostname) {
         /* This tries to identify the valid syntaxes for the our synthetic "gateway" host. */
         return STRCASE_IN_SET(hostname, "_gateway", "_gateway.");
 }
-
-int sethostname_idempotent(const char *s);
-
-int shorten_overlong(const char *s, char **ret);
-
-int read_etc_hostname_stream(FILE *f, char **ret);
-int read_etc_hostname(const char *path, char **ret);