Fix psql's \copy command to ensure that it cycles libpq back to the idle state

Fix psql's \copy command to ensure that it cycles libpq back to the idle state
(in particular, causing the ReadyForQuery message to be eaten) before
returning from do_copy.  The only known consequence of failing to do so is
that get_prompt might show a wrong result for the %x transaction status
escape, as reported by Bernd Helmle; but it's possible there are other issues.

Back-patch as far as 7.4, the oldest version supporting %x.