x11/lightdm-gtk-greeter: and x11/slick-greeter: auto-set greeter-session

Add pkg-install and pkg-deinstall scripts to automatically configure
the greeter-session variable in lightdm.conf if it has not been manually
changed from the default value.

This reduces the steps for a fresh FreeBSD install to get to a graphical
environment to the following simple steps (excluding graphics drivers):
$ pkg install <greeter> <desktop-environment>
$ service lightdm enable
$ service ligthdm start

Approved by: ericbsd (maintainer x11/slick-greeter)
Approved by: madpilot (maintainer x11/lightdm-gtk-greeter)

Differential Revision: https://reviews.freebsd.org/D31632