pgsql: Fix psql's \copy command to ensure that it cycles libpq back to

[tgl(at)postgresql(dot)org (Tom Lane)]

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/bin/psql:
        copy.c (r1.70 -> r1.71)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/copy.c.diff?r1=1.70&r2=1.71)