pg_dump: Reduce dependencies on global variables.

Change various places in the code that are referencing the global
Archive object g_fout to instead reference the Archive object fout
which is already being passed as a parameter.  For parallel pg_dump to
work, we're going to need multiple Archive(Handle) objects, so the
real solution here is to pass down the Archive object to everywhere
that it needs to go, but we might as well pick the low-hanging fruit
first.