Attach update hooks in the network admin. Accounts for the rare situation...

Attach update hooks in the network admin. Accounts for the rare situation where the network admin is not the main site. props johnbillion, see #18876.

git-svn-id: https://core.svn.wordpress.org/trunk@19021 1a063a9b-81f0-0310-95a4-ce76da25c4cd