Running mysql_upgrade should end up with the exact same system tables as fresh
installations have after running mysql_install_db. To ensure the upgrade is
correct and complete:

- Remove the redundant modification of thread_id`. On 5.5 version, the
  `general_log` table was created as `CREATE TABLE IF NOT EXISTS general_log
  (..., thread_id INTEGER NOT NULL, ...)`, and starting from 10.0+, the table is
  created as `CREATE TABLE IF NOT EXISTS general_log (..., thread_id BIGINT(21)
  UNSIGNED NOT NULL, ...)`, but mysql_upgrade is not properly upgrading the
  table. It modifies the `thread_id` twice in one query, which could leave the
  table not modified and lead to other potential error when upgrading from
  MariaDB 5.5 or older.

- Update `servers` to ensure `Host` and `User` has correct data type if
  upgrading from 10.1 or older. On versions 10.0 and 10.1, the `servers` table
  was created as `CREATE TABLE IF NOT EXISTS servers (..., Host char(64) NOT
  NULL DEFAULT , ..., Owner char(64) NOT NULL DEFAULT , ...)`, and starting
  from 10.2, the table is created as `CREATE TABLE IF NOT EXISTS servers (...,
  Host varchar(2048) NOT NULL DEFAULT , ..., Owner varchar(512) NOT NULL
  DEFAULT , ...)`.

All new code of the whole pull request, including one or several files that
are either new files or modified ones, are contributed under the BSD-new license.
I am contributing on behalf of my employer Amazon Web Services, Inc.