#2308 many issues with defint of atan2

[Stavros Macrakis]

integrate(atan2(x,0),x,-1,1) => expt: undefined: 0 to a negative exponent.

integrate(atan2(x,x),x,-1,1) => %pi/2, but
integrate(atan2(x,x),x,-1,0) => -3*%pi/4
integrate(atan2(x,x),x,0,1) => %pi/4
... so presumably should be -%pi/2

integrate(atan2(0,x),x,-1,1) => noun form
... but can do -1..0 and 0..1

Presumably the problem is that defint doesn't do anything clever with piecewise-defined functions.