-f <output file> option for pg_dumpall

In pgAdmin we use pg_dump's -f option to write backup files. The IO
streams are redirected to display status and errors etc. in the GUI.

In order to enhance the interface to allow backup of entire clusters as
well as role and tablespace definitions, we need to be able to get
pg_dumpall to write it's output directly to a file in the same way,
because we cannot redirect the child pg_dump IO streams (which also
means we may miss errors, but I need to think about that some more).

As far as I can see, adding a -f option to pg_dumpall should be straight
forward, the only issue being that we'd need to pass pg_dump an
additional (undocumented?) option to tell it to append to the output
file instead of writing it as normal.

Any thoughts or better ideas?

Regards, Dave.