Provide a C vararg interface to gsubr invocation.

Provide a C vararg interface to gsubr invocation.

* libguile/eval.i.c (CEVAL): Update calls to `scm_i_gsubr_apply ()' with
  a fixed number of arguments.  Use `scm_i_gsubr_apply_list ()' for
  calls with a list of arguments of unknown length.
  (SCM_APPLY): Use `scm_i_gsubr_apply_list ()' instead of
  `scm_i_gsubr_apply ()'.

* libguile/gsubr.c (gsubr_apply_raw): New.
  (scm_i_gsubr_apply): Change to take a C vararg list instead of a
  Scheme list.  Use `gsubr_apply_raw ()'.
  (scm_i_gsubr_apply_list): Use `gsubr_apply_raw ()'.

* libguile/gsubr.h (scm_i_gsubr_apply): Update prototype.
  (scm_i_gsubr_apply_list): New declaration.