Cleanup and improvements to linuxdvb frontend statistics

... , especially added fallbacks to old API in case new API does not provide prop value.

For me,  new API is available, call to FE_GET_PROPERTY succeeds, but all fe_properties[*].u.st.len are zero. Old API, however, works just fine. Thus, this patch adds fallbacks to old API in case something goes wrong with the new API.