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.

Additionally, lots of improvements and fixes in the linuxdvb statistics code and in the web ui were done.