lightdm: Set umask to 007

The current umask for desktop applications is 022, which is the
default umask for systemd services and is inherited down from
lightdm.service to the desktop applications.

This lax umask with o=r was seen to be a problem.  We have a
recommended value of 007 for the umask in /project/admin/skel/.bashrc.

However, applications started from the desktop don't have this
recommended value, because the users .bashrc is not read by default.

Whe tried to read the users .bashrc during desktop initalization, but
had to revert that because of problems with Anaconda, which installs
incompatible junk into the users path.

Set umask of lightdm.service to 007.

lightdm.be0

@@ -89,6 +89,7 @@ EOF
 	ExecStart=/usr/bin/lightdm
 	StandardOutput=syslog
 	Restart=always
+	UMask=007
 
 	[Install]
 	WantedBy=graphical.target