Use libgcrypt instead of standalone AES and SHA code

[pete4abw]

I realize that libgcrypt recently had a big bug in 1.9.0, but I would like to look at using its code to replace AES and SHA code snippets. Comments?

EDIT: libgcrypt can also be used for MD5 and CRC. This will eliminate the need for multiple source files and provide a generalized interface for all hash/encryption activities.

EDIT: Issues. Compatibility with existing archives. For example, cannot use SHA3 or alternate hash because archive hashes are fixed. Decrypt will be impossible.

BUT otoh using a generalized library will allow for user-selected hash and encryption.

This will take some work and lots of investigation.

[pete4abw]

libgcrypt has been implemented.