zonectl: display conventional zones better during RZ

zonectl's Report Zones subcommand displays a tabular list of zones. A
conventional zone's WP column is displayed as 0xffffffffffffffff , the
literal value that the HDD reports. But that's too wide for the column,
causing the text to be misaligned. It's also not really meaningful,
because the Write Pointer isn't really defined for a Conventional zone.
Change it to "-1" to fix the text misalignment.

MFC after: 2 weeks
Sponsored by: ConnectWise
Reviewed by: fuz
Differential Revision: https://reviews.freebsd.org/D57512