Add trim support to zpool wait

Manual trims fall into the category of long-running pool activities
which people might want to wait synchronously for. This change adds
support to 'zpool wait' for waiting for manual trim operations to
complete. It also adds a '-w' flag to 'zpool trim' which can be used to
turn 'zpool trim' into a synchronous operation.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: John Gallagher <john.gallagher@delphix.com>
Closes #10071

include/sys/fs/zfs.h

@@ -1353,6 +1353,7 @@ typedef enum {
 	ZPOOL_WAIT_REMOVE,
 	ZPOOL_WAIT_RESILVER,
 	ZPOOL_WAIT_SCRUB,
+	ZPOOL_WAIT_TRIM,
 	ZPOOL_WAIT_NUM_ACTIVITIES
 } zpool_wait_activity_t;