Don't overwrite scan key in systable_beginscan()

Don't overwrite scan key in systable_beginscan()

When systable_beginscan() and systable_beginscan_ordered() choose an
index scan, they remap the attribute numbers in the passed-in scan
keys to the attribute numbers of the index, and then write those
remapped attribute numbers back into the scan key passed by the
caller.  This second part is surprising and gratuitous.  It means that
a scan key cannot safely be used more than once (but it might
sometimes work, depending on circumstances).  Also, there is no value
in providing these remapped attribute numbers back to the caller,
since they can't do anything with that.

Fix that by making a copy of the scan keys passed by the caller and
make the modifications there.

Also, some code that had to work around the previous situation is
simplified.

Discussion: https://www.postgresql.org/message-id/flat/f8c739d9-f48d-4187-b214-df3391ba41ab@eisentraut.org