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/sysusers/sysusers.c

@@ -33,12 +33,13 @@
 #include "hashmap.h"
 #include "path-util.h"
 #include "selinux-util.h"
+#include "smack-util.h"
 #include "specifier.h"
+#include "string-util.h"
 #include "strv.h"
 #include "uid-range.h"
 #include "utf8.h"
 #include "util.h"
-#include "smack-util.h"
 
 typedef enum ItemType {
         ADD_USER = 'u',