Use E'' strings internally only when standard_conforming_strings =

Use E'' strings internally only when standard_conforming_strings =
'off'. This allows pg_dump output with standard_conforming_strings =
'on' to generate proper strings that can be loaded into other databases
without the backslash doubling we typically do.  I have added the
dumping of the standard_conforming_strings value to pg_dump.

I also added standard backslash handling for plpgsql.