strv: add strv_free_free() to strv.c and make use of it

Let's teach it a new trick, and make it return NULL.

src/basic/strv.h

@@ -154,3 +154,5 @@ static inline bool strv_fnmatch_or_empty(char* const* patterns, const char *s, i
         return strv_isempty(patterns) ||
                strv_fnmatch(patterns, s, flags);
 }
+
+char ***strv_free_free(char ***l);