Try to be a bit more strict about integer types, use off_t or int64_t for
file size and file offsets.
Replace safe_ato*() by safe_atoi32() and safe_atoi64(), fix integer types
at call sites to match.
This parser actually supports way more than is needed for RSP proper,
as mt-daapd was piggybacking on the RSP queries for its smart playlists.
As I don't have (yet?) the RSP specs, better safe than sorry here. This
will be revisited at some point. Or not.