add FreeBSD support, use module data

[fraenki]

• Add support for the FreeBSD operating system
• Replace params.pp with module data
• Update metadata.json to reflect current state of this module
• Small updates to README.md: remove unused section, add hint for postfix on CentOS
• Update copyright/authors information
• Small style fixes

I'd suggest to release a new version if you decide to accept this PR :)

[oxc]

@fraenki there is no mailx package on Ubuntu 16.04, this is only a virtual package.

Package mailx is a virtual package provided by:
s-nail 14.8.6-1
mailutils 1:2.99.99-1ubuntu2
bsd-mailx 8.1.2-0.20160123cvs-2

I do not like the default configuration raise an error here. Looks like before, the module selected bsd-mailx. Can you add some Debian-specific config to reflect the previous state?

See https://github.com/oxc/puppet-postfix/pull/1/files#r166102379

Since we don't need/want to support ancient Debian versions, I believe choosing bsd-mailx as default for Debianoids is fine, do you agree?

[oxc]

Selecting bsd-mailx on newer versions got lost in the change.

[oxc]

I'd suggest to release a new version if you decide to accept this PR :)

0.2.1 is now on puppet forge