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.

We should free the nid and in such a case we were doing it.

netapi32: Downgrade a FIXME and an ERR to a trace to not clutter test output on handled error conditions.

This is a preparation for using multiple contexts on one drawable to
handle multithreading.