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.

doc/ref/api-modules.texi

@@ -945,14 +945,18 @@ the @var{name} is not bound in the module, signals an error.  Returns a
 variable, always.
 
 @example
+static SCM eval_string_var;
+
+/* NOTE: It is important that the call to 'my_init'
+   happens-before all calls to 'my_eval_string'. */
+void my_init (void)
+@{
+  eval_string_var = scm_c_public_lookup ("ice-9 eval-string",
+                                         "eval-string");
+@}
+
 SCM my_eval_string (SCM str)
 @{
-  static SCM eval_string_var = SCM_BOOL_F;
-
-  if (scm_is_false (eval_string_var))
-    eval_string_var =
-      scm_c_public_lookup ("ice-9 eval-string", "eval-string");
-
   return scm_call_1 (scm_variable_ref (eval_string_var), str);
 @}
 @end example