Have \copy go through SendQuery

This enables a bunch of features, notably ON_ERROR_ROLLBACK.  It also
makes COPY failure (either in the server or psql) as a whole behave more
sanely in psql.

Additionally, having more commands in the same command line as COPY
works better (though since psql splits lines at semicolons, this doesn't
matter much unless you're using -c).

Also tighten a couple of switches on PQresultStatus() to add
PGRES_COPY_BOTH support and stop assuming that unknown statuses received
are errors; have those print diagnostics where warranted.

Author: Noah Misch