util-lib: split our string related calls from util.[ch] into its own file string-util.[ch]

There are more than enough calls doing string manipulations to deserve
its own files, hence do something about it.

This patch also sorts the #include blocks of all files that needed to be
updated, according to the sorting suggestions from CODING_STYLE. Since
pretty much every file needs our string manipulation functions this
effectively means that most files have sorted #include blocks now.

Also touches a few unrelated include files.

src/basic/json.c

@@ -19,9 +19,11 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <sys/types.h>
 #include <math.h>
+#include <sys/types.h>
+
 #include "macro.h"
+#include "string-util.h"
 #include "utf8.h"
 #include "json.h"