Allow pool names that look like Solaris disk names

Nothing bad happens if a prefix of your pool name matches a disk name.
This is a bit of a silly restriction at this point.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #11781
Closes #11813