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/log.c

@@ -19,26 +19,28 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <stdarg.h>
-#include <stdio.h>
 #include <errno.h>
-#include <unistd.h>
 #include <fcntl.h>
+#include <printf.h>
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdio.h>
 #include <sys/socket.h>
 #include <sys/un.h>
-#include <stddef.h>
-#include <printf.h>
+#include <unistd.h>
 
 #include "sd-messages.h"
-#include "log.h"
-#include "util.h"
-#include "missing.h"
-#include "macro.h"
-#include "socket-util.h"
+
 #include "formats-util.h"
+#include "macro.h"
+#include "missing.h"
 #include "process-util.h"
-#include "terminal-util.h"
 #include "signal-util.h"
+#include "socket-util.h"
+#include "string-util.h"
+#include "terminal-util.h"
+#include "util.h"
+#include "log.h"
 
 #define SNDBUF_SIZE (8*1024*1024)