fix #H7618 - gas cloud affects underwater monsters

Stinking cloud placed near water or poison gas breathed across it
would affect and potentially kill underwater monsters.  Most swimmers
are on the surface and should be affected, but eels and other fish
shouldn't be.

This also changes minliquid() to not treat flying and levitating as
ways to survive water when on the Plane of Water.

I think goodpos() needs to be taught about that Plane (where many ways
of existing at a water location don't apply).  This doesn't do that.

doc/fixes36.2

@@ -225,6 +225,7 @@ scattering of objects might leave source location with wrong thing displayed
 for configurations with 'long int' larger than 'int', lev_comp wrote some
 	garbage into the *.lev files, but nethack seemed unaffected by that
 	(at least on little-endian hardare) and loaded the levels successfully
+stinking cloud placed near water could kill underwater creatures
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository