pg_dump: Add ExecuteSqlCopy and simplify ExecuteSqlQuery

The thread about tablecmds.c being too large for some code inspection
tooling got me thinking about ways to improve the second worst offender:
pg_dump.c.

Those investigations of mine haven't yet revealed a way to split the file
that I find effective, let alone helpful. However, it has shown me a few
patterns within pg_dump.c that I'm proposing to change.

Of the current 84 calls to the convenience routine ExecuteSqlQuery(), 83 of
them are regular queries which expect a result of PGRES_TUPLES_OK. The
other one expects PGRES_COPY_OUT. So it seems worthwhile to remove the
parameter entirely and assume the caller wants PGRES_TUPLES_OK. For the one
exception, ExecuteSqlCopy() is created, though one could argue that we
might just do without the convenience routine altogether. Patch attached.