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.