Improve 'zpool import' EBUSY error message

When a device is already open O_EXCL by another process the
`zpool import` will correctly fail.  However, the default failure
message isn't very helpful.  It may in fact be harmful if you
take its advise and destroy your pool.

    cannot import 'tank': pool is busy
            Destroy and re-create the pool from
            a backup source.

Improve the error message in the EBUSY case to simply print a
message indicating that the devices are current in use.  The user
will need to manually identify which process has the device open
exclusively and why.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

lib/libzfs/libzfs_pool.c

@@ -1585,6 +1585,12 @@ zpool_import_props(libzfs_handle_t *hdl, nvlist_t *config, const char *newname,
 			(void) zpool_standard_error(hdl, error, desc);
 			break;
 
+		case EBUSY:
+			zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
+			    "one or more devices are already in use\n"));
+			(void) zfs_error(hdl, EZFS_BADDEV, desc);
+			break;
+
 		default:
 			(void) zpool_standard_error(hdl, error, desc);
 			zpool_explain_recover(hdl,