WL#10353: Use utf8mb4 in libmysql and command tools

Libmysql uses a default compiled in character set for when auto-detecting the
environment settings fails. 
And it will switch to utf8mb3 for environments that use UTF8. 
On top of the some client tools will do their own inference and set the
connection charset according to the outcome of it. 

This worklog is about switching the compiled in default to utf8mb4 and making
sure that both libmysql's and client tools' autodetection picks utf8mb4 instead
of utf8mb3.

User Documentation
==================

* https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-2.html
* https://dev.mysql.com/doc/refman/8.0/en/charset-connection.html
* https://dev.mysql.com/doc/refman/8.0/en/mysql-real-connect.html

This WL takes care of changing the default charset for client tools from latin1
to utf8mb4.
Client tools include following: mysql, mysqladmin, mysqldump, mysqlcheck, 
mysqlpump, mysqlimport, mysql_upgrade, mysqltest, mysqlshow, mysqlslap.

New mysqltest option default-character-set:
------------------------------------------
As of today mtr tests are run with latin1 as default charset. With this
WL, mysqltest client (this is the client used by mtr) default charset is changed
to utf8mb4, which makes ~6K test files to be run with utf8mb4 charset. This
change will need a lot of result files to be re-recorded after some analysis
(80% will be a blind record). So to reduce the scope of this WL to minimal result 
file updates this WL introduces a new option --default-character-set in mysqltest 
client, and set to latin1 in default-client.cnf.

Note: default-character-set option exists for all clients except mysqltest.

Changes to default-client.cnf:
------------------------------

All clients (except mysqltest) default charset in default-client.cnf will be
changed to utf8mb4 to ensure that all clients will run with utf8mb4 as default.

[mysqltest]
default-character-set=latin1

Above new option will be added to default-client.cnf to ensure that mtr tests 
pass.