oleaut32: Clear the input pvar structure in VARIANT_UserUnmarshal to free the data instead of just initializing.

Rework storage to make all its interactions with the underlying data go 
though ReadAt and WriteAt to allow for an ILockBytes underlying layer.
Implement ReadAt and WriteAt in stg_bigblockfile.c for blockfiles.

Reject invalid flags in DTM_SETSYSTEMTIME i.e. when the flag is neither
GDT_VALID nor GDT_NONE when the style is set to DTS_SHOWNONE.  Corresponding
setters and message sequence tests were also added.