* dcrt0.cc (jit_debug): New global.

* dcrt0.cc (jit_debug): New global.
(initial_env): Set jit_debug when we are automatically starting a gdb process.
* dtable.cc (dtable::get_debugger_info): Don't tty tricks when we are being
debugged by our own captive gdb, as determined by jit_debug == true.
(dtable::init_std_file_from_handle): Detect errors when initializing a tty
early rather than at random points later.
* fhandler.h (fhandler_*::init): Return int to indicate success/failure.
* fhandler.cc (fhandler_base::init): Reflect change in return value.
* pipe.cc (fhandler_pipe::init): Ditto.
(fhandler_pipe::create_selectable): Don't say we're retrying when we aren't.
* fhandler_console.cc (fhandler_console::init): Ditto.  Return success/failure.
* fhandler_serial.cc (fhandler_serial::init): Ditto.
* fhandler_tty.cc (fhandler_tty_slave::init): Ditto.
(fhandler_tty_slave::open): Make debugging output more detailed.
* tty.cc (tty_list::terminate): Don't close I/O handles before all slaves have
checked in.
(tty::slave_alive): Make a non-inlined function.  Check if tty pipe handles can
be created as an additional exists check.
* tty.h (tty::slave_alive): Just define here.