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/debug-generator/debug-generator.c

@@ -23,6 +23,7 @@
 #include "strv.h"
 #include "unit-name.h"
 #include "mkdir.h"
+#include "string-util.h"
 
 static const char *arg_dest = "/tmp";
 static char **arg_mask = NULL;