[nsp] Interface up a time interval

[atticus at satanic.org]

> If it's sufficient for you to make sure no traffic is forwarded outside
> that time, you can use a time-based access-list.

This is definately the right way to go about this, but don't forget about:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123newft/123_1/g_kron.htm

...which along w/ the exposure of tclsh probably qualify as the top two
potentially-useful-but-inevitably-encourages-wrong-solutions features in
12.3/12.3T.