kvm_close(3): return `error` instead of blindly returning `0`

`error` is the accumulated error from previous close(2) calls.

This bug has been present since the libcall's import from 4.4BSD
Lite (r1573).

MFC after:	1 week
Noticed by:	vangyzen (D10022)
Relnotes:	yes
Sponsored by:	Dell EMC Isilon

lib/libkvm/kvm.c

@@ -292,7 +292,7 @@ kvm_close(kvm_t *kd)
 		free(kd->pt_map);
 	free((void *)kd);
 
-	return (0);
+	return (error);
 }
 
 int