don't include my_progname in the error message, my_error starts from it
automatically, resulting in, like

/usr/bin/mysqladmin: Notice: /usr/bin/mysqladmin is deprecated and will be removed in a future release, use command 'mariadb-admin'

and remove "Notice" so that the problem description would directly
follow the executable name.

make the check to work when the executable is in the PATH
(so, invoked simply like 'mysql' and thus readlink cannot find it)

fix the check in mysql_install_db and mysql_secure_installation to not
print the warning if the intermediate path contains "mysql" substring

add this message also to
* mysql_waitpid
* mysql_convert_table_format
* mysql_find_rows
* mysql_setpermissions
* mysqlaccess
* mysqld_multi
* mysqld_safe
* mysqldumpslow
* mysqlhotcopy
* mysql_ldb

Closes #2273