File data decryption verifies a 16 bit checksum and the unpacked
data contains a 22 byte signature. That's more than enough to
identify the file format without relying on the extension.

There were no checks for the size of the input data in decrypt()
and unpack(). Add them, along with the following changes:
* Add inputsize parameter to unpack() (needed for the checks).
* Pass whole input data to unpack() (including decryption header).
* Pass output length to unpack_block(), drop oend from the class
  and make unpack() and unpack_block() static methods.
* Use size_t where appropriate.
* Add an enum constant headersize for the size of the decryption
  header.
* Clean up the unpack code a little bit.
* Remove now unused CHARP_AS_WORD macro.

* Set last byte of song name and instrument names to NUL to
  ensure string termination.

* Refuse to load file when order/insruments/pattern number in
  the header is too big to avoid writing past the end of the
  arrays while loading data. (Note that we can use only 255 orders
  because of "orders[header.ordnum] = 0xFF;". If there exist
  files with 256 orders that needs to change.)

The loaded data is not sanity checked, crashes during playback
can still happen. But in order to know what needs to be checked,
Cs3mPlayer has to be fixed first.