DB2 ODBC FDW PostgreSQL extension

I reinvigorated my Postgresql DB2 wrapper. Now it works also with PostgreSQL 11 and 12. The source code and installation instruction are available here.

It took me some time to accomplish it. The PostgreSQL failed because of memory violation related problem after returning from GetForeignPaths function. The crashed happened inside the server code. What's more, it worked after recompiling the server directly from source code so nailing down the problem was impossible. Finally, the solution was to migrate the db2odbc_fdw.c to pure C code and get rid of all warnings. Probably the reason for the failure was a discrepancy between data types in C function signature and launching sequence.