math: add isnan() function

[jow-]

Add a new isnan() convenience function to the math library which can be used to test if a given value is a NaN double.

The same test can be realized without the math library by using a function similar to the following one:

function isNaN(x) {
    return x != x;
}

Signed-off-by: Jo-Philipp Wich jo@mein.io