skeleton: Rename /etc/profile.d/umask to umask.sh

/etc/profile only sources files that matches the /etc/profile.d/*.sh
pattern, so /etc/profile.d/umask was never sourced.

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>