refs #74 enhance kdbgscan performance for win8/2012 x64

traditionally, kdbgscan has been used for two reasons: to determine a
suggested profile or to gather information used to speed up other
plugins. this patch optimizes kdbgscan when its used for the later
purpose. specifically, when you call kdbgscan with —profile (thus you
must already know its win8/2012 x64), it will skip the typical
signature-based KDBG scan. this reduced wyatt’s execution time from 30
minutes to 6 minutes on a 140 GB dump.