add support for specifying 32 or 64-bit registry access

This adds an extra parameter to most of the post/windows/registry
methods called 'view' that specifies if a registry key should be
accessed as a native process, 32-bit or 64-bit.

Support is added to both the Meterpreter and command-line backends. For
the command backend, a lot of boilerplate is removed from each method in
favor of a few shared commands. There is an error hash that never gets
used, so I removed it as well.

This passes the post/test/registry module with meterpreter, but fails
the command line backend. However, it fails in the same way without
these changes (tested on Windows 8), so I suspect that the command line
session was already not working well, at least with newer versions of
Windows. I might look into figuring out how to fix that, but it looks
pretty fragile to me, parsing for english phrases in the output.

Author: Brent Cook