Make better use of existing enums in plpgsql

Make better use of existing enums in plpgsql

plpgsql.h defines a number of enums, but most of the code passes them
around as ints.  Update structs and function prototypes to take enum
types instead.  This clarifies the struct definitions in plpgsql.h in
particular.

Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>