MFH: r438362

x11/lightdm: Use correct replacement for clearenv()

The previous patch to replace the Linux clearenv(3) function did not
actually clear the environment, but instead created a new environment
variable named environ with the value "NULL".

PR: 218564
Reported by: jbeich
Obtained from: FreeBSD env(1) command

Approved by: ports-secteam (junovitch)