interactive_context: construct with `new' instead of ad-hoc

interactive_context: construct with `new' instead of ad-hoc

keypress_handler: rename `ctx' to `I'

meta_x: first arg is now buffer instead of window

command-line: -uu changed to +u

command-line: -f and +u use get_recent_conkeror_window() instead of window_watcher

command-line: do not pass config/cwd to new windows

Normal command-line switches (other than -q and -E) can be invoked with a + instead
of a - character.  This allows us to use command-line switches like `u' which Mozilla
otherwise consumes.