Fix up pg_dump to do string escaping fully correctly for client encoding

and standard_conforming_strings; likewise for the other client programs
that need it.  As per previous discussion, a pg_dump dump now conforms
to the standard_conforming_strings setting of the source database.
We don't use E'' syntax in the dump, thereby improving portability of
the SQL.  I added a SET escape_strings_warning = off command to keep
the dumps from getting a lot of back-chatter from that.