strv: convert strv_split_quotes into a generic strv_split_extract

strv: convert strv_split_quotes into a generic strv_split_extract

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.