Try again to resolve problems with mistaking sessions for windows: now

Try again to resolve problems with mistaking sessions for windows: now
do not look up windows as sessions (and panes as windows) when they are
qualified with a ':' or a '.'. So 'foo' as a window target will look for
windows and sessions called 'foo', but ':foo' will only look for
windows, and 'foo:' only for sessions. This means the common case of
using an unadorned session as a window target (send -tfoo) should
continue to work, but an explicit window will not get confused with a
session (send -t:foo).