Fix 'zpool add' safety check corner cases

Three cases were discovered where 'zpool add' would fail to
warn when adding vdevs to a pool with a mismatched replication
level. These are:

1. When a pool contains mixed file and disk vdevs.
2. When a pool contains an active dRAID distributed spare
3. When a pool contains an active hot spare

The lack of warnings are caused by get_replication() assessing
the current pool configuration an inconsistent and disabling
the mismatched replication check for the new pool configuration
after 'zpool add'. This change updates get_replication() to
be slightly more tolerant in the non-fatal case.

The zpool_add_010_pos.ksh test case was split in to separate
tests: zpool_add_warn_create.ksh, pool_add_warn_degraded.ksh,
and zpool_add_warn_removal. These test were extended to
include coverage for dRAID pools and the three scenarios
described above.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #17780