[OS X Emacs] Emacs 25 Mojave fixes backport

[David Reitter]

I back-ported the two macOS Mojave fixes to Emacs 25.
Please check my handiwork before I push it [1]

One user reported issues with the cursor leaving traces while scrolling [2], which I reproduced on a pre-Mojave machine, but not on my Mojave system.  I’m unclear whether that is also the case on the 26 branch.  I can try tomorrow unless someone else can comment.

[1] https://github.com/davidswelt/aquamacs-emacs/tree/emacs25-mojave
[2] https://www.dropbox.com/s/5rtt22lg0ddy07w/Aquamacs%20cursor.mov?dl=0