/etc/profile: read in /etc/profile.d/*.sh files

For custom projects, it is more maintainable to add custom profile settings
in a separate file, than directly in /etc/profile.

This patch modifies /etc/profile to read in *.sh files from /etc/profile.d/,
a technique commonly used in Linux distributions.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>