Replace multi-signature functions with named parameters and defaults

[pramsey]

As discussed at the 2018 code sprint, we should bite the bullet and retire all the multi-signature SQL wrapped functions with simple named parameters and default values. We can also take this opportunity to use named parameters in all our signatures as well, to improve flexibility of parameter ordering.

[pramsey]

Started on this. One side effect: going to totally drop ST_AsKML(version int, geog) signature, since the version is always required to be '2'. That's dumb, and if anyone uses that signature, they can just stop.

[pramsey]

Pretty much done at r17300. A global pass to use named parameters is a separate task.