Allow add of raidz and mirror with same redundancy

Allow new members to be added to a pool mixing raidz and mirror vdevs
without giving -f, as long as they have matching redundancy. This case
was missed in #5915, which only handled zpool create.

Add zfstest zpool_add_010_pos.ksh, with test of zpool create
followed by zpool add of mixed raidz and mirror vdevs.

Add some more mixed raidz and mirror cases to zpool_create_006_pos.ksh.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Haakan Johansson <f96hajo@chalmers.se>
Issue #5915
Closes #6181