pg_dump large file support

I've implemented large file support for pg_dump, in what I hope is a
portable fashion. Please review the attached patch.

This needs an additional option from autoconf, because pg_dump prints a
hex offset which needs to be either %Lx or %llx, but there is no hex
equivalent to INT64_FORMAT. To get this working I have arbitrarily
defined OFF_T_FORMATX as "%lld", but that needs to be changed before the
patch is incorporated.

Peter E suggested that there was no need for the macro definitions in
pg_backup.h but I wasn't clear how else to achieve what was needed.

--
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight, UK
http://www.lfix.co.uk/oliver
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