[imp] access to IMP w/o a horde generated login screen / alternate_login

[Liam Hoekenga]

> The initial launch of IMP is just fine.  It looks like the alternate_login
> doesn't get invoked when you logout of IMP, and then hit the browser's back
> button from whatever logout screen you're at (horde, or redirect_at_logout).

I put some debugging stubs, and in IMP 3.2.2 if you use the browser's back
button from the logout screen, the resulting call to login does invoke the
alternate_login.

alternate_login does not get invoked in HEAD under the same circumstances.

Liam