These files do not contain sensitive data, so keep the default files
permissions.

sendmail(1) reads parts of it's configuration from the main.cf file.
When the file is not readable, the command invocation will fail with the
following error:

fatal: open /etc/postfix/main.cf: Permission denied

The parameters read from main.cf are documented here:
http://www.postfix.org/sendmail.1.html

It is an optional array of strings containing addtional entries for the
master.cf file

Make sure our version of the lens gets installed

* Fix resource dependencies

Dependencies must be set only if the resources exist in the catalog.
For some weird reason, this worked in Puppet 4 (and shouldn't have).

* Fix AbsolutePath test for Puppet 5

To be able to configure master.cf with chroot

* Use data types

* Use data types

* Use data types

* Use data types

* Use data types

* Use data types

* Use data types

* Include puppet version requirements in metadata

* Support Ruby 2.3.1

This patch is needed to support the new Puppet v4 AIO which uses Ruby
2.3.1.

Fix params validation + specs

* Move params validation to corresponding class: We only need to valide the
params when they need to be set / are actually used. Since Puppet 4.6 or 4.7,
stdlib's validate_string function doesn't accept undef anymore, so this has
to be changed.

* Remove specific error checking in specs: types checking validate_* functions
have different error messages depending on the version the stdlib module, the
version of puppet, maybe the version of ruby, the phase of the Moon... who knows.