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.