minor nit after auth-dialog removal

[Ray Kohler]

I see that you're gearing up for a major new release here. Very nice!
Hopefully it'll be soon, as I've been using Chaskiel's multiple
password patch since he created it, and it'd be great to see it in a
real release.

I do see one very small Makefile bug right now - after the auth-dialog
is gone, the install target probably shouldn't be creating
/usr/libexec any longer, since nothing here needs it. (Also, how about
installing the manpage during make install? Or was it a deliberate
decision not to do that?)