config: user: check for <aio.h>

And always zpool_read_label_slow() on non-conformant libcs

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: José Luis Salvador Rufo <salvador.joseluis@gmail.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13207
Closes #13254