(#838) Fixed installer skipping dialog

I have identified the problem with the installer skipping the PHP INI level dialog. (http://trac.i-mscp.net/ticket/838)

The reason is the condition when the dialog is shown. And in normal install mode (no reconfigure or preseed) this is only done when $self::apacheConfig{'INI_LEVEL'} is not set or incorrect. But the default config already sets it and therefore the dialog is skipped.

Unfortunately I discovered a further problem in the installer. Using the back button forces the dialog to be shown because reconfigure is set. But if you go back let's say 10 dialogs and then hit OK again, all the 10 dialogs are skipped forward. That's because of a similar condition used in most of the dialogs which does not display it when the value is already set.

I tried to fix this problem too by changing the conditions when the dialog is set.
I identifed 3 conditions for this:
- in reconfigure mode
- not set in old config
- incorrectly set

You can see the fix already in this commit.

If that's ok I would like to modify the remaining installers in a similar way and include this in this pull request.

Please let me know if I overlooked something and how/if I should proceed. Thx :)