input/chronovu_la8: only send data to the session, don't send the header

The file format is funny, a data part is leading (fixed size) and a
"header" part follows. The previous implementation sent the header part
to the session, too, pretending it was part of the data. This change
limits the number of samples that get sent to the session.

Comment on the file layout and header fields while we are here. This
information got lost in commit 02e24c0ce0d8 when the input module got
converted from do-it-yourself file operations to having intrinsic
handler routines invoked from common logic which handles the file.