cleanup: copy string in taxonomy_set_[country|category]

These functions were taking a const char *, yet taking ownership
of the value. Moreover, taking ownership of strings is rather
unusual in C-style APIs. Let's copy the string instead.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/taxonomy.c

@@ -105,7 +105,7 @@ void taxonomy_set_category(struct taxonomy_data *t, enum taxonomy_category categ
 		}
 		idx = t->nr++;
 	}
-	t->category[idx].value = value;
+	t->category[idx].value = strdup(value);
 	t->category[idx].origin = origin;
 	t->category[idx].category = category;
 }