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.
strv_split_extract is to strv_split_quotes as extract_first_word was to
unquote_first_word.
Now there's extract_first_word for extracting a single argument,
extract_many_words for extracting a bounded number of arguments,
and strv_split_extract for extracting an arbitrary number of arguments.
