ECPG: Start tracking subtransactions and cursors in the runtime library.

Introduce 4 new public functions: ECPGopen(), ECPGfetch(), ECPGcursor_dml()
and ECPGclose().

Use the previously exposed ecpg_do() to drive all these functions.

New error handling is added to mimic the server behaviour if
a previous function checking the savepoint or cursor name failed.

The arguments to ECPGtrans() have changed.
The library version was changed to 7.0 from 6.6 because of this.