#24 Blackmarket shouldn't generate potential pets

[Silas Dunsmore]

Presented for discussion and possible inclusion in Slash'EM.

I believe it is too easy to get a pet in the black market. The player can
simply wait for a domestic-type monster to be generated, or just zap a
wand of create horde a few times. Then tame the animal as usual.

This works around that, making it an actual effort to get a pet. (I can
think of three or four ways, and there are probably more.)

This patch applies cleanly to Slashem 0.0.8E0F1.

The new code in this patch is contributed by the author, Silas Dunsmore,
to the Slash'EM project, and to the general Nethack community, under the
terms of the Nethack General Public License.

[Silas Dunsmore]