comctl32/monthcal: Control elements should be centered in client area, and left/top aligned if they don't.

This allows to load them with LOAD_LIBRARY_AS_DATAFILE.

When mode is STGM_WRITE or STGM_READWRITE make sure key key exists.
Only read data in STGM_READ or STGM_READWRITE mode.
Do not depend on value type being REG_BINARY.
Succeed when reading not existing values.

Add comments to functions that are not implemented.

On Write holes between file size and cursor are filled with 0 bytes.
On SetSize holes caused by increasing file size are filled with 0 bytes.