knsupdate: add @ creates $origin, del @ deletes \@.$origin

$ knsupdate
knsupdate> zone asdf
knsupdate> origin asdf
knsupdate> add @ txt upa
knsupdate> del @ txt upa
knsupdate> show
Update query:
;; ->>HEADER<<- opcode: UPDATE; status: NOERROR; id: 2319
;; Flags: ; ZONE: 1; PREREQ: 0; UPDATE: 2; ADDITIONAL: 0

;; UPDATE SECTION:
asdf.                   3600    IN      TXT     "upa"
\@.asdf.                0       NONE    TXT     "upa"

This isn't just a display bug, it does actually try to delete \@.$origin.

Compare nsupdate.bind9:

$ nsupdate.bind9
> zone asdf
> add @ 10 txt upa
> del @ txt upa
> show
Outgoing update query:
;; ->>HEADER<<- opcode: UPDATE, status: NOERROR, id:      0
;; flags:; ZONE: 0, PREREQ: 0, UPDATE: 0, ADDITIONAL: 0
;; ZONE SECTION:
;asdf.                          IN      SOA

;; UPDATE SECTION:
.                       10      IN      TXT     "upa"
.                       0       NONE    TXT     "upa"

(admittedly, @ is not useful there at all since it doesn't have origin).

Happens on Debian 3.2.6-1 and 3.3.1-1.