MFC r256331:

Add $VAR_ZFSINTERACTIVE (zfsInteractive) and new f_zfsinteractive() for
determining when a script wants to be nonInteractive but selectively
wants ZFS operations to be *interactive* (this is analgous to already
existing $VAR_NETINTERACTIVE (netInteractive) and f_netinteractive()
used for the same purpose (script wants to be nonInteractive but wants
network operations to be *interactive*).

Approved by:	re (glebius)