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

src/backend/libpq/auth.c

@@ -1037,6 +1037,10 @@ static GSS_DLLIMP gss_OID GSS_C_NT_USER_NAME = &GSS_C_NT_USER_NAME_desc;
 #endif
 
 
+/*
+ * Generate an error for GSSAPI authentication.  The caller should apply
+ * _() to errmsg to make it translatable.
+ */
 static void
 pg_GSS_error(int severity, const char *errmsg, OM_uint32 maj_stat, OM_uint32 min_stat)
 {
@@ -1227,7 +1231,7 @@ pg_GSS_recvauth(Port *port)
 		{
 			gss_delete_sec_context(&lmin_s, &port->gss->ctx, GSS_C_NO_BUFFER);
 			pg_GSS_error(ERROR,
-						 gettext_noop("accepting GSS security context failed"),
+						 _("accepting GSS security context failed"),
 						 maj_stat, min_stat);
 		}
 
@@ -1253,7 +1257,7 @@ pg_GSS_recvauth(Port *port)
 	maj_stat = gss_display_name(&min_stat, port->gss->name, &gbuf, NULL);
 	if (maj_stat != GSS_S_COMPLETE)
 		pg_GSS_error(ERROR,
-					 gettext_noop("retrieving GSS user name failed"),
+					 _("retrieving GSS user name failed"),
 					 maj_stat, min_stat);
 
 	/*
@@ -1317,6 +1321,11 @@ pg_GSS_recvauth(Port *port)
  *----------------------------------------------------------------
  */
 #ifdef ENABLE_SSPI
+
+/*
+ * Generate an error for SSPI authentication.  The caller should apply
+ * _() to errmsg to make it translatable.
+ */
 static void
 pg_SSPI_error(int severity, const char *errmsg, SECURITY_STATUS r)
 {