* libguile/null-threads.h: Reimplement null-threads stubs for pthread
  data types, initializers, and functions in terms of types and inline
  functions instead of CPP macros.  Fixes unused-value warnings, and
  tightens things up in general.
  (scm_i_pthread_cleanup_push, scm_i_pthread_cleanup_pop): Remove these,
  as they were unused and incorrect -- they would never run the cleanup
  handler even if 1 was passed to pop.