Issue #1534688 by johan.wilfer, generalredneck: Remove forced line breaks between message array elements.

Issue #1272248 by mrded: Remove h3 tags from fieldset titles on settings page as they break some custom themes.

Issue #1324980: Move mailsystem settings to the module configuration/test page so that a test mail may be sent via other MailSystemInterface implementations.

Issue #1166986 by Domsou: Correct $headers['Return-Path'] to $message['headers']['Return-Path'] in HMTLMailSystem::mail()

Improved hook_requirements() and hook_enable() to ensure that HTMLMailSystem shows up on Mail System settings page.

Issue #131737: Ensure that the Return-Path is set when sending mail on both Windows and non-Windows systems.