Support PKZIP Master Key

We might want to first decide on the general issue of supporting (or not) cracking of non-passwords in JtR. For example, we could also want to add cracking of raw DES, like hashcat already has, but perhaps not only on GPU, but also on FPGA. That said, here goes:

There's this recent paper about PKZIP cracking on GPU, which gives plenty of credit to our work and shows that a GPU implementation of what we already do on CPU is worthwhile (we might want to create a separate GitHub issue for that):

http://www.insticc.org/node/TechnicalProgram/icissp/presentationDetails/73605

There's also this recent addition to hashcat:

https://github.com/hashcat/hashcat/pull/2032
https://twitter.com/hashcat/status/1129441728761610242

where the Master Key is cracked by having it effectively reduced from 96 to just 48 still-unknown bits, if I understand correctly.

No source file, test, or entry point is named. First read the existing CPU PKZIP implementation and the linked hashcat references, then resolve whether non-password cracking and a PKZIP Master Key implementation are in scope; completion would require an agreed design and corresponding implementation tests.