* syscalls.cc (setsid): Detach process from its console if the current

* syscalls.cc (setsid): Detach process from its console if the current
controlling tty is the console and already closed.
* dtable.h (class dtable): Add members to count descriptors referring to the
console.
* dtable.cc (dtable::dec_console_fds): New function to detach process from its
console.
(dtable::release): Decrement the counter of console descriptors.
(dtable::build_fhandler): Increment it.
* exception.cc (ctrl_c_handler): Send SIGTERM to myself when catch
CTRL_SHUTDOWN_EVENT.