x11/lightdm: Update to 1.30.0, fix crash and change default config

This update addresses a crash in lightdm, related to the way the
user environment for the lightdm process is setup in src/process.c.

The configuration option in lightdm.conf for locked memory is now
honoured - the default configuration has been updated to disable this,
and pkg-message has been updated to reflect this change.

Changes this release:

https://github.com/CanonicalLtd/lightdm/releases/tag/1.30.0

PR: 238083
Submitted by: madpilot
Reported by: David.Boyd49@twc.com
Differential Revision: https://reviews.freebsd.org/D20541