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/env-util.c

@@ -550,7 +550,7 @@ char **replace_env_argv(char **argv, char **env) {
                         if (e) {
                                 int r;
 
-                                r = strv_split_quoted(&m, e, EXTRACT_RELAX);
+                                r = strv_split_extract(&m, e, WHITESPACE, EXTRACT_RELAX|EXTRACT_QUOTES);
                                 if (r < 0) {
                                         ret[k] = NULL;
                                         strv_free(ret);