Fix PQencryptPasswordConn to work with older server versions.

 Fix PQencryptPasswordConn to work with older server versions.
 
 password_encryption was a boolean before version 10, so cope with "on" and
 "off".
 
 Also, change the behavior with "plain", to treat it the same as "md5".
 We're discussing removing the password_encryption='plain' option from the
 server altogether, which will make this the only reasonable choice, but
 even if we kept it, it seems best to never send the password in cleartext.