Add support for UPDATE modify actions.

This adds support for UPDATE DML into the Informix FDW. The
modify action will reuse an updatable cursor established during the
ForeignScan phase to update the target row by using WHERE CURRENT OF.

This commit introduces all the missing basic infrastructure for UPDATE,
the next commits will add all required datum conversion routines. Only
INT8, INT4 and INT2 are there for testing purposes.

Also introduce basic regression tests for INSERT, DELETE and UPDATE
actions on a remote table.