Use mysqli in db_url for Drupal 6, fixes #1325

[rfay]

The Problem/Issue/Bug:

OP#1372 points out that our standard drupal 6 config in settings.ddev.php uses the mysql:// db_url, but php 7.2 with D6LTS wants the mysqli:// db_url.

How this PR Solves The Problem:

This just changes "mysql://" to "mysqli://" in Drupal6 settings.ddev.php. It doesn't seem to affect traditional Drupal 6 configuration.

Manual Testing Instructions:

• Configure d6 traditional project, verify that it works
• Configure D6LTS project using PHP 7.2, verify that it works.
• Disable php-mbstring in the web container to solve the problem discussed below:
  • phpdismod php-mbstring
  • killall -1 php-fpm
  • At this point you should be able to install d6lts

Details about php-mbstring.
Note that there is a problem with D6LTS that I don't totally understand at install time. It insists:

Multibyte string output conversion in PHP is active and must be disabled. Check the php.ini mbstring.http_output setting. Please refer to the PHP mbstring documentation for more information. (Currently using Unicode library Error)

However, in our post-php-5.6 configurations we do not have mbstring.http_output set:

$ php -i | grep http_output
mbstring.http_output => no value => no value

However, you can import a d6 db and it lets you walk past this problem.

Automated Testing Overview:

Related Issue Link(s):

OP #1325

Release/Deployment notes:

[andrewfrench]

I'm able to successfully install D6 and D6LTS (on PHP 5.6), but the steps included still lead to an error while installing on PHP7.2.

$ ddev config
...
$ ddev config --php-version 7.2
$ ddev start
$ ddev ssh 
$ phpdismod php-mbstring 
$ killall -1 php-fpm

mbstring.http_output => no value => no value

 Multibyte string input conversion in PHP is active and must be disabled. Check the php.ini mbstring.http_input setting. Please refer to the PHP mbstring documentation for more information. (Currently using Unicode library Error)