Made ntfsdecrypt compatible with libgrypt-1.6

Made ntfsdecrypt compatible with libgrypt-1.6

The upcoming libgrypt-1.6 drops the "module" interface which was used
by ntfsdecrypt for decrypting files which were encrypted with the "DESX"
algorithm. This algorithm is a Microsoft variant of DES with a key size
of 128 bits, and is not natively supported by libgrypt. The module interface
made possible to declare an external algorithm so that all the encryption
modes could be processed the same way whether the algorithm was internal
or external.
This patch makes DESX a specific case, so that the module interface is
not needed any more. It is compatible with current libgrypt and upcoming
libgrypt-1.6