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/socket-proxy/socket-proxyd.c

@@ -38,6 +38,7 @@
 #include "path-util.h"
 #include "set.h"
 #include "socket-util.h"
+#include "string-util.h"
 #include "util.h"
 
 #define BUFFER_SIZE (256 * 1024)