libzutil: optimize zpool_read_label with AIO

Read all labels in parallel instead of sequentially.

Originally committed as
https://cgit.freebsd.org/src/commit/?id=b49e9abcf44cafaf5cfad7029c9a6adbb28346e8

Obtained from: FreeBSD
Sponsored by: Spectra Logic, Axcient
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alek Pinchuk <apinchuk@axcient.com>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #11467