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/modules-load/modules-load.c

@@ -21,13 +21,14 @@
 
 #include <errno.h>
 #include <getopt.h>
+#include <libkmod.h>
 #include <limits.h>
 #include <string.h>
 #include <sys/stat.h>
-#include <libkmod.h>
 
 #include "conf-files.h"
 #include "log.h"
+#include "string-util.h"
 #include "strv.h"
 #include "util.h"