Keep dh_fixperms from cleaning up our mode changes

dh_fixperms will try to clean everything up to reasonable permissions,
but we really want to keep the ones we made in override_dh_install. Add
an override for dh_fixperms to do nothing.

[endlessm/eos-shell#2718]
Merge pull request #1 from endlessm/shell/2718

Keep dh_fixperms from cleaning up our mode changes