Fix thread-unsafe lazy initializations.

Fix thread-unsafe lazy initializations.

* libguile/debug.c (scm_local_eval):
  libguile/ports.c (scm_current_warning_port):
  libguile/strports.c (scm_eval_string_in_module): Perform
  lazy-initialization while holding a mutex.  Use SCM_UNDEFINED as the
  uninitialized value.  Use 'scm_c_*_variable'.

* doc/ref/api-modules.texi (Accessing Modules from C): Fix
  'my_eval_string' example to be thread-safe.