Here's the post-Step-1 commit.  It works for multiple banlist windows.

Note particularly what I've done to banlist.h.
Note that for many functions in banlist.c the argument is now
a banlist_info *, rather than a session *.
Note in banlist.c the initialization of array modes[] which
contains driving information for the checkboxes.

Of course those checkboxes aren't yet implemented.  Maybe in Step 2
I will change to checkboxes from radio buttons; but definitely I will
change to letting modes[] drive processing.