stand/boot1.efi: Implement bootonce for ZFS

Implement ZFS bootonce protocol. We pass zfs-bootonce=t to the next boot
stage as a command line argument. Unlike zfsboot -> loader handoff in
the BIOS case, we don't use the OS_BOOTONCE_USED. This would require
modifications to loader.efi which would only server to make it more
complicated. Instead, use the command line parsing interface for the
boot1.efi -> loader.efi to pass in the zfs-bootonce kenv that will be
needed by rc.d/zfsbe to activate the BE if boot progresses that far.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D39412