Add the ability to purge unmanaged settings in inifile

[c33s]

currently the php.ini files are loaded with comments and settings which are default. it would be a good thing to be able to purge these, like it is already possible for fpm-pools

currently ini_settings from puppetlabs/inifile only allow purging for child-providers. there is also a open issue for adding the feature to make purging easier.

an existing solution for this is also hash2ini function which got rejected from puppetlabs-stdlib

edit: also see the related issue for adding support for "specify owner, group, or permission"