Fix detector tube item counting

A detector tube would sometimes miss a decrement, due to the pos structure
passed to can_go being modified by the tubed-item entity while it's also
sitting in the timer queue as an argument to item_exit.  As the can_go
API actually depends on being able to mutate the pos structure (see the
teleporter tube), we can't just start passing in a never-mutated copy
of pos.  So for now just fix in the detector tube's can_go, by copying the
structure there.  Incidentally, make the decrement mechanism consistent
between the two forms of increment.
Don't spill an extra pick from old node breakers

Due to historical shenanigans around the addition of the pick slot to
node breakers, some breakers that were placed before the addition have
ended up with a mese pick in their newly-acquired pick slot, rather than
in the ghost_pick slot where it belongs.  This causes these breakers to
spill this ghost pick when they are broken.  To avoid this, make pick
spilling conditional on the breaker having a formspec: pre-slot breakers
don't have one.
Set infotext for node breakers

As node breakers now have a form, they should also have infotext to draw
attention to it, as do most form-containing nodes.
Don't show legacy auto-tree-tap in creative invy