dof: number of degrees of freedom
tails: 1 or 2
The survival function is 1 minus the cumulative distribution function.
This function is Excel compatible for non-negative x.
If dof < 1 this function returns a #NUM! error. If tails is neither 1 or 2 this function returns a #NUM! error. The parameterization of this function is different from what is used for, e.g., NORMSDIST. This is a common source of mistakes, but necessary for compatibility.