dracut: support mountpoint=legacy for root dataset

Support mountpoint=legacy for the root dataset in the dracut zfs support
scripts.

mountpoint=/ or mountpoint=/sysroot also works.

Change zfs-env-bootfs.service to add zfsutil to BOOTFSFLAGS only for
root datasets with mountpoint != legacy.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Rafael Kitover <rkitover@gmail.com>
Closes #15149