ecpg array support, or lack thereof

While looking at the memory leaks in ecpg that Coverity warned about and
Michael just fixed, I started wondering if the code is ever used.

The leaks were in the code that takes a host variable, and converts it
into a string for sending to the server as a query parameter. In
particular, it was in code that deals with arrays. However, the fine
manual says:

> SQL-level arrays are not directly supported in ECPG. It is not
> possible to simply map an SQL array into a C array host variable.
> This will result in undefined behavior. Some workarounds exist,
> however.

That very clearly says that the code that was fixed is not actually
supported.

We do have a test case, 'arrays', that tests the code, but it only tests
integer arrays. The leaks were in 'interval', 'timestamp', and 'numeric'
arrays. And it turns out that there are two bugs there:

1. In timestamp, interval, and date, the array offset is calculated
incorrectly:

str = quote_postgres(PGTYPESinterval_to_asc((interval *) ((var +
var->offset * element)->value)), quote, lineno);

That "var + var->offset * element" has C datatype "struct variable *",
not "char *" as the code assumes, so the calculated offset is wrong,
leading to bogus parameters or segfault

2. The constructed array looks like this (for timestamp):

array [2005-06-23 11:22:33,2004-06-23 11:22:33]

That's bogus. It's sent as a query parameter, not embedded into an SQL
string, so the syntax should be '{...}'.

It would be nice to fix these, and add a test case to cover all kinds of
arrays. Although technically, it works as advertised, because the manual
says that it will result in "undefined behaviour".

- Heikki