remove --purge php5-mysql/php5-mysqlnd delete the same configurations

[jasonz]

Ubuntu 14.04 LTS

Reproduce:

```
$ sudo apt-get install php5-fpm php5-mysql
$ ls /etc/php5/mods-available
json.ini mysql.ini mysqli.ini mysqlnd.ini opcache.ini pdo.ini pdo_mysql.ini
$ ls /etc/php5/fpm/conf.d
05-opcache.ini 10-mysqlnd.ini 10-pdo.ini 20-json.ini 20-mysql.ini 20-mysqli.ini 20-pdo_mysql.ini
$ sudo apt-get install php5-mysqlnd
$ dpkg -l | grep php5-mysql
rc php5-mysql
ii php5-mysqlnd
$ sudo apt-get remove --purge php5-mysql
$ ls /etc/php5/mods-available
json.ini mysql.ini mysqli.ini mysqlnd.ini opcache.ini pdo.ini pdo_mysql.ini
$ ls /etc/php5/fpm/conf.d
05-opcache.ini 10-mysqlnd.ini 10-pdo.ini 20-json.ini 20-pdo_mysql.ini
```

Replace php5-mysql with php5-mysqlnd, then `apt-get remove --purge php5-mysql` will delete /etc/php5/fpm/conf.d/20-mysql.ini and /etc/php5/fpm/conf.d/20-mysqlnd.ini.

But those two soft linked configurations now belongs to php5-mysqlnd which I still need.
This behavior will lead to php child process crash in some certain cases.
The user needs to `ln -s` to recreate the soft link.

I'm not sure if it is the intended behavior.

Maybe rename those two ini files will solve the problem?
Or the user need to `apt-get remove --purge php5-mysql` first, then `apt-get install php5-mysqlnd`.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.