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

poudriere testport = ok
pkg install lightdm-gtk-greeter -> confirm lightdm.conf is edited: greeter-session=lightdm-gtk-greeter
pkg install slick-greeter -> confirm lightdm.conf not edited (already changed from default)
pkg remove slick-greeter -> confirm lightdm.conf not edited (already changed from default)
pkg remove lightdm-gtk-greeter -> confirm lightdm.conf is edited #greeter-session=example-gtk-gnome
pkg install slick-greeter -> confirm lightdm.conf is edited: greeter-session=slick-greeter
pkg install lightdm-gtk-greeter -> confirm lightdm.conf not edited (already changed from default)
pkg remove lightdm-gtk-greeter -> confirm lightdm.conf not edited (already changed from default)
pkg remove slick-greeter -> confirm lightdm.conf is edited #greeter-session=example-gtk-gnome