Fix set of NLS translation issues

Fix set of NLS translation issues

While monitoring the code, a couple of issues related to string
translation has showed up:
- Some routines for auto-updatable views return an error string, which
sometimes missed the shot.  A comment regarding string translation is
added for each routine to help with future features.
- GSSAPI authentication missed two translations.
- vacuumdb handles non-translated strings.
- GetConfigOptionByNum should translate strings.  This part is not
back-patched as after a minor upgrade this could be surprising for
users.

Reported-by: Kyotaro Horiguchi
Author: Kyotaro Horiguchi
Reviewed-by: Michael Paquier, Tom Lane
Discussion: https://postgr.es/m/20180810.152131.31921918.horiguchi.kyotaro@lab.ntt.co.jp
Backpatch-through: 9.3