UDF: Minor code cleanup

Immediately store pointers as new pointer type rather than using
intermediate uint8_t pointer.

Also "unneed" some of the "needed" pointers as soon as we're able to
release them rather than holding on until the end of the UDF image.

Add assorted debug messages and code comments.

Make FileSetDescriptor optional as minor step towards supporting
ExtendedFileEntries.

Minor variable name changes for readability.

Use tag_identifier enum for variable type rather than uint16_t and
add "INVALID_DESCRIPTOR" (0) to enum and use it in the switch. This way
we're not comparing enums with ints.

Move GenericVolumeStructureDescriptor to udf.h.