emacs: fix nil file-name-handler-alist after reloading init

This problem would cause emacs to not treat files like *.gpg
or *.el.gz especially before displaying them, as it normally does.

So now, I update a/file-name-handler-alist with the latest contents of
file-name-handler-alist before loading the init file, and afterwards I
restore things back to normal (done in a/post-init).