Let `scm_mkstrport' allocate buffers on the caller's behalf.

Let `scm_mkstrport' allocate buffers on the caller's behalf.

* libguile/strports.c (INITIAL_BUFFER_SIZE): New macro.
  (scm_mkstrport): If STR is false, allocate a bytevector on the
  caller's behalf.
  (scm_object_to_string, scm_call_with_output_string,
  scm_open_output_string): Pass SCM_BOOL_F as the STR argument of
  `scm_mkstrport'.

* libguile/backtrace.c (scm_display_application,
  display_backtrace_body): Likewise.

* libguile/gdbint.c (scm_init_gdbint): Likewise.

* libguile/print.c (scm_simple_format): Likewise.