fix use after free when loading an about:* page from about:about. Reported by Brian Callahan, thanks!

Save closed tabs to a "killed tabs" queue, for up to a max of "max-killed-tabs", and allow to re-open them. Keep the tab history but not its content. This integrates nicely with the session file, just add the "killed" flag as alias for TAB_KILLED and put it in the ktabshead queue upon initalization. It's thus possible to re-open closed tabs even after telescope has been closed, and tabs keeps their history! Add a new cmd, cmd_tab_undo_close, to try to re-open the most recently closed tab and bind it to `u'. While here, add a TODO for the tab history memory leak.

"set set-title = 1" is quite a mouthful. The old name will still be supported for a while to ease backwards compatibility.

On RET, execute the command on the selected line rather than what the user typed. This saves a TAB (to expand the entry) and is more in line with the other completion commands.