Add radius to spherical functions (MySQL/MariaDB inaccurate earth radius)

[khooz]

Hi,

I recently tried spatial features of MySQL and MariaDB and found this strange constant for default earth radius. Searching how is that possible, I found this thread on GIS StackExchange. It seems the default radius on spherical functions are the result of some lazy copy-pasting from an old piece of code and consequently "inaccurate" by around 20 meters or so.
That, we can't change, but we can provide proxies for ST functions with all their parameters, required and optional. It'd be more flexible and anyone who enjoys control and accuracy can benefit from those extra options.

Thanks