Problem:    v:windowid isn't set on MS-Windows.
Solution:   Set it to the window handle. (Chris Sutcliffe)

Problem:    maparg() doesn't return the flags, such as <buffer>, <script>,
            <silent>.  These are needed to save and restore a mapping.
Solution:   Improve maparg(). (also by Christian Brabandt)

Problem:    Can't pass the X window ID to another application.
Solution:   Add v:windowid. (Christian J. Robinson, Lech Lorens)

Documentation updates.