bug#35968: Release 26.3

[npostavs]

Tracking bugs to fix before releasing Emacs 26.3.

I'm thinking of the following:

#35767 26.1; byte compiler lost warning about calling define-key with wrong 
number of arguments
       - regression from 25.3, has a simple fix.
       - https://debbugs.gnu.org/35767#8
#29220 26.0.90; eieio-persistent-read fail to restore saved object.
       - regression from 25.3, reasonably simple partial (conditional on 
eieio-backward-compatibility) fix.
       - https://debbugs.gnu.org/29220#193
#35739 [w32] Bad signature from GNU ELPA for archive-contents
       - fixed in master
       - for emacs-26 need something less invasive
#35770 [PATCH] Broken duplicate case elimination in switch byte-compilation
       - regression from 25.3, fixed in master
       - for emacs-26 just disable optimization (?)