Set umask in xkb's install-system and uninstall-system scripts

[ghost]

If the umask is set to something like 077 in /etc/profile for hardening purposes, running ./install-system.sh creates files unreadable by the user. Furthermore, on my system they also changed the permissions of the $xkbdir/rules/evdev file, which my DE did not like very much.
My suggestion would be to set the umask inside the generated scripts meant for linux.