ismindex: Rename structs and fields from "file" to "track"

The tool nowadays supports more than one track per file,
this makes reading the code slightly less confusing.

Signed-off-by: Martin Storsjö <martin@martin.st>
ismindex: Factorize code for printing chunk duration lists

Signed-off-by: Martin Storsjö <martin@martin.st>
ismindex: Check the return value of allocations

Signed-off-by: Martin Storsjö <martin@martin.st>