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/path-util.c

@@ -19,21 +19,22 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <string.h>
-#include <unistd.h>
 #include <errno.h>
-#include <stdlib.h>
-#include <stdio.h>
 #include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 #include <sys/statvfs.h>
+#include <unistd.h>
 
-#include "macro.h"
-#include "util.h"
-#include "log.h"
-#include "strv.h"
-#include "path-util.h"
-#include "missing.h"
 #include "fileio.h"
+#include "log.h"
+#include "macro.h"
+#include "missing.h"
+#include "string-util.h"
+#include "strv.h"
+#include "util.h"
+#include "path-util.h"
 
 bool path_is_absolute(const char *p) {
         return p[0] == '/';