bsdinstall: zfsboot: Prompt user for zpool name if the pool already exists

If one install FreeBSD on multiple disks (say 13 and CURRENT) the first created
pool will always be used.
Prompt the user for a new pool name if we detect that the default or supplied one
already exists.

Reviewed by: imp
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33331