Update ext/pgsql parameter names

[nikic]

Because of how pervasive it is in the API, I retained the $field terminology (rather than $column).

[kocsismate]

Is it possible to get rid of the $stmt names here? I think we only use it in ext/mysqli, and nowhere else.

[kocsismate]

And what's your stance on renaming the stmt parameters? Or do you want them to be consistent with mysqli?

[nikic]

And what's your stance on renaming the stmt parameters? Or do you want them to be consistent with mysqli?

I've switched things to use $statement_name now.