Add macro `GetCtlDPage` for Control Manager

[digarok]

Apple IIgs Toolbox Reference Volume 1 has an error on page 4-60. It has the title of the page as GetCtlDpage but for the C call below it shows the correct capitalization GetCtlDPage (capital P).

Also, I verified this is how it is use in Apple's code like this snippet from Dialog Manager:

    pha
    _GetCtlDPage
    pld

Proposal: add this other capitalization style to Library/Ctl.Macs.s, but leave the existing capitalization to avoid breaking anything.