followup: drinking from sinks while levitating

followup: drinking from sinks while levitating
This is a followup to the patch I made a couple months ago.  It replaces
the "!Levitation && !u.uswallow" checks with can_reach_floor(), which
makes a more complete set of checks and is more consistent.  I applied
this to fountains too.  I doubt that fountains hit the ceiling, and the
checks also seem reasonable for heights in between (e.g. while mounted).