Handle (i.e. do not increase) zero-width windows.

The `:windo` command enters each window, which causes the height to increase to at least 1 (to make the current line visible). (The so-called "Rolodex mode" uses stacks of 0-width windows, with only one maximized.)
To avoid changes to the window layout, it (and the current window number) need to be saved and restored after the `:windo`. This affects the iteration in `s:check_special_tabpage()` (during `:SessionSave`) as well as the commands added to the session (on `:SessionOpen` restoring a single tab page).