Backport security fixes for x11/sddm

The 0.18 release of x11/sddm contains a fix for a security error
that probably doesn't affect us: session-reuse. In any case our
default configuration is not vulnerable. This doesn't update to
0.18 because there's a bunch of other changes that would need to
be chased, further delaying this update.

While here, pet portlint and Tijl, who asked for a pkg-message.

PR: 230029
Reported by: doctorwhoguy@gmail.com
Security: f00acdec-b59f-11e8-805d-001e2a3f778d