msr_init will initialize a MSRecord structure. If the msr parameter is NULL a new structure will be
allocated. If the msr parameter is not NULL the blockette chain (MSRecord.blkts) will be freed but any
memory allocated for MSRecord.fsdh and MSRecord.datasamples will be preserved as it will cleanly be re-
used by routines such as msr_unpack(3).
msr_free will free all memory associated with a MSRecord structure including the blockette chain and set
the structure pointer (*ppmsr) to 0.
msr_free_blktchain will free all memory associated with the blockette chain of an MSRecord structure.
The shortcut blockette pointers will also be reset.