x11/sddm: Update to 0.18.1.
Upstream does not provide a config file anymore. SDDM still reads
/usr/local/etc/sddm.conf if it is present, but the intended way to configure
it now is using /usr/local/etc/sddm.conf.d/ directory. See man sddm.conf(5) for
details.
Also include a patch to make SDDM take /etc/login.conf into account.
PR: 241519
Submitted by: Martin Birgmeier <d8zNeCFG@aon.at> (login.conf patch)
Differential Revision: https://reviews.freebsd.org/D23579