strv: multiple cleanups

- turn strv_merge into strv_extend_strv.
   appending strv b to the end of strv a instead of creating a new strv
- strv_append: remove in favor of strv_extend and strv_push.
- strv_remove: write slightly more elegant
- strv_remove_prefix: remove unused function
- strv_overlap: use strv_contains
- strv_printf: STRV_FOREACH handles NULL correctly

src/sysctl/sysctl.c

@@ -251,13 +251,9 @@ static int parse_argv(int argc, char *argv[]) {
                                 if (*p == '.')
                                         *p = '/';
 
-                        l = strv_append(arg_prefixes, optarg);
-                        if (!l)
+                        if (strv_extend(&arg_prefixes, optarg) < 0)
                                 return log_oom();
 
-                        strv_free(arg_prefixes);
-                        arg_prefixes = l;
-
                         break;
                 }