Obvious optimization for EIGRP format: Move some work to
get_salt, where it's done once instead of millions times
per second.
Support meet-in-the-middle hint for Office 97/2003 hashes.
Speedup on CPU is about 2x but it doesn't show in benchmark.
See #736.

We don't actually have the tool for finding the mitm hint
(yet) - oclHashcat has.
Fix plugin headers dependency in Makefile for john.o.
Shared OpenCL autotune tweaks.