Rationale:

  - With TLS, SCM_I_CURRENT_THREAD is never NULL but the whole struct,
    including ‘guile_mode’, is zero when the thread starts.

  - Without TLS, SCM_I_CURRENT_THREAD is NULL until ‘guilify_self_1’ has
    been called.

Thoughts?

(My “benchmark” has been running for some time already and the bug
hasn’t shown up again.)

Thanks,
Ludo’.