modules: add OnAddNetwork, OnDeleteNetwork hooks.

    <KiNgMaR> why is there no OnDeleteNetwork hook?
    <kylef_> KiNgMaR because I never added one
    <kylef_> and no one else cared :P
    <kylef_> KiNgMaR PR welcome :)
    <kylef_> and OnAdd
    <KiNgMaR> it's necessary
    <KiNgMaR> someone complained that identfile can be crashed otherwise
    <KiNgMaR> and I'm facing exactly the same problem in win32 identserver module
    <KiNgMaR> I will look at how OnDeleteUser is implemented and see if I can come up with a PR

There could possibly be something very wrong with this patch, although it does seem to work. Thanks for looking into it!