nut: refactor upssched uci-defaults script

nut: refactor upssched uci-defaults script

Add checks not to overwrite defaultnotify options in the nut-sendmail-notify fashion.
Use lists for defaultnotify instead of option.
Add check not to overwrite notifycmd if already defined.
upssched-cmd script must not be called directly, it is called by the upssched binary with needed arguments.

Signed-off-by: Pascal Coudurier <coudu@wanadoo.fr>