Disable Internet Sharing -- check command might be broken but fix works

[kristovatlas]

I verified on my El Capitan machine that the following fix in the YAML works:
defaults write /Library/Preferences/SystemConfiguration/com.apple.nat NAT -dict-add Enabled -bool false

However, the check command seems broken. I allowed internet sharing on one of my networking interfaces, and the /Library/Preferences/SystemConfiguration/com.apple.nat file doesn't exist. (Maybe it would if I re-logged or rebooted.)

I recommend just performing the defaults read portion of the check.