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.

src/basic/strv.h

@@ -73,7 +73,7 @@ static inline bool strv_isempty(char * const *l) {
 char **strv_split(const char *s, const char *separator);
 char **strv_split_newlines(const char *s);
 
-int strv_split_quoted(char ***t, const char *s, ExtractFlags flags);
+int strv_split_extract(char ***t, const char *s, const char *separators, ExtractFlags flags);
 
 char *strv_join(char **l, const char *separator);
 char *strv_join_quoted(char **l);