Don't abort windows position remembering if small playlist size is invalid at the beginning (Closes:#557)

Transcode cropping/padding patch by Udo Richter < udo underscore richter at gmx d0t de> (with added round-to-16 fix) . Many thanks for this.

Seems to work neatly, please test nevertheless.

Closes:#448

- Code simplification

* rpn.c: add "services_discovery_add", "services_discovery_remove" and "services_discovery_is_loaded" rpn functions.
* macro.c, http.h, mvar.c: add new "object" param2 for foreach macro. It gives a list of all objects matching a capability.