libzfs: zpool_load_compat(): open feature file cloexec

As a bonus, this also passes the open flags into the open flags instead
of the mode (it worked by accident because O_RDONLY is 0),
correctly detects a failed map,
and prefaults the entire file since we're always writing to every page

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11993