Adding basic support for mounting multiple files

The multifile feature currently takes a "config" directory as the
mountpoint, with a system file and whatever other registry files it can
find, and lists them all under /HKLM.  (In the long run it should be
able to take a Windows installation directory and load some other
hives, too.)  The behavior when mounting a single file is unchanged.
Expanded multifile support

Hivekey names are now separate from registry file names, to better match
what regedit shows in Windows.  Also, the default file is loaded as
HKU\.DEFAULT.
Restructured unit tests

Redundant code is consolidated and required example files are all
defined in one place.
fixed incorrect file stats in test definition
fixed buggy test_key() for multifile test case