Det-rule error

The det-rule didn't work properly. First bug was that the relex2logic-rules.txt file called the method choose_var_name which was named choose-var-name in relex-to-logic.scm.
There's another error now saying that choose-var-name is returning an argument of the wrong type (?).
```
37: 4 [lp (det-rule "man" # choose-var-name ...)]
In ice-9/eval.scm:
 368: 3 [eval # #]
In unknown file:
   ?: 2 [cog-new-node VariableNode {#<procedure choose-var-name ()>}]
In ice-9/boot-9.scm:
 102: 1 [#<procedure a75bee0 at ice-9/boot-9.scm:97:6 (thrown-k . args)> wrong-type-arg ...]
In unknown file:
   ?: 0 [catch-closure wrong-type-arg "cog-new-node" ...]

ERROR: In procedure catch-closure:
ERROR: In procedure cog-new-node: Wrong type argument in position 2 (expecting string name for the node): #<procedure choose-var-name ()>
ABORT: wrong-type-arg
```
I've had a look at the functions, but couldn't find the error yet as random-string which is called by choose-var-name is returning a string. Maybe you can help, @amebel?