Do not print anything when self-backtrace unavailable

Right now, gdb's self-backtrace feature will still print something
when a backtrace is unavailable:

   sig_write (_("----- Backtrace -----\n"));
[...]
     sig_write (_("Backtrace unavailable\n"));
    sig_write ("---------------------\n");

However, if GDB_PRINT_INTERNAL_BACKTRACE is undefined, it seems better
to me to print nothing at all.

This patch implements this change.  It also makes a couple of other
small changes in this same module: it adds a header guard to
bt-utils.h, and it protects the definitions of
gdb_internal_backtrace_1 with a check of GDB_PRINT_INTERNAL_BACKTRACE.