Report pool suspended due to MMP

When the pool is suspended, record whether it was due to an I/O error or
due to MMP writes failing to succeed within the required time.

Change spa_suspended from uint8_t to zio_suspend_reason_t to store the
reason.

When userspace queries pool status via spa_tryimport(), report the
reason the pool was suspended in a new key,
ZPOOL_CONFIG_SUSPENDED_REASON.

In libzfs, when interpreting the returned config nvlist, report
suspension due to MMP with a new pool status enum value,
ZPOOL_STATUS_IO_FAILURE_MMP.

In status_callback(), which generates and emits the message when 'zpool
status' is executed, add a case to print an appropriate message for the
new pool status enum value.

Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Giuseppe Di Natale <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Olaf Faaland <[email protected]>
Closes #7296 

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