prop: prop_sbuf / prop_ptr cleanups

prop: prop_sbuf / prop_ptr cleanups

The prop get string callbacks should use static buffers. This
patch is trying to fix the problematic parts (mostly in dvr_db.c)
and also introduces prop_ptr to not allocate static pointer
in each string get callback.

The problems in dvb_db.c were serious, so tvh could crash or
invalid data are written to the config files.

Issue: #4636