* debug.cc (mark_closed): Rename from debug_mark_closed and make static.

* debug.cc (mark_closed): Rename from debug_mark_closed and make static.
(setclexec_pid): New function for marking saved handle as close-on-exec.
(delete_handle): New function.
(debug_fixup_after_fork): New function.
* debug.h: Declare new functions, remove obsolete ones.
* fork.cc (debug_fixup_after_fork): Call to cleanup close-on-exec handles.
* fhandler.cc (fhandler_disk_file::close): Minor reorg.
(fhandler_base::set_inheritance): Set flag appropriately for debugging when
close-on-exec so forked process can delete closed handles.
* tty.h (open_output_mutex): Eliminate unneeded argument.
(open_input_mutex): Ditto.
* fhandler_tty.cc (fhandler_tty_slave::open): reflect open_*_mutex argument
changes.
* fhandler.h (fhandler_socket): Make saw_shutdown_* functions type bool.
* tty.cc (tty::get_event): Eliminate unneeded argument.
(tty::common_init): Reflect change to get_event.  Events should always be
inherited.