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/env-util.c

@@ -22,11 +22,12 @@
 #include <limits.h>
 #include <unistd.h>
 
+#include "def.h"
+#include "string-util.h"
 #include "strv.h"
 #include "utf8.h"
 #include "util.h"
 #include "env-util.h"
-#include "def.h"
 
 #define VALID_CHARS_ENV_NAME                    \
         DIGITS LETTERS                          \