fix github issue #125 - grave gold not buried

fix github issue #125 - grave gold not buried

Fixes #125

When a random grave included some gold among whatever treasure was
generated, that gold was left on top of the grave instead of being
buried inside it like other treasure.

I'm sure this was intentional but only because mkgold() puts the
gold on the ground and merges it with other gold if there is already
some present.  Keeping an existing stack of gold distinct from the
new one in order to bury the latter is feasible but clumsy.  Just
make a new gold object directly, bypassing mkgold(), and bury that.