* infrun.c (stop_soon_quietly): Make it an enum, to better
override the default behavior of handle_inferior_event.
(clear_proceed_status): Update uses of stop_soon_quietly to
reflect that it is now an enum.
(start_remote): Ditto.
(handle_inferior_event): Change logic a bit if stop_soon_quietly
is set to handle the new GNU/Linux kernel behavior for
attach/sigstop. Update uses of stop_soon_quietly.
* inferior.h (enum stop_kind): New enum.
* infcmd.c (attach_command): Use STOP_QUIETLY_NO_SIGSTOP.
Reset normal handle_inferior_event behavior, afterwards.
* fork-child.c (startup_inferior): Update.
* alpha-tdep.c (heuristic_proc_start): Update.
* solib-svr4.c (svr4_solib_create_inferior_hook): Update.
* solib-sunos.c (sunos_solib_create_inferior_hook): Update.
* solib-osf.c (osf_solib_create_inferior_hook): Update.
* solib-irix.c (irix_solib_create_inferior_hook): Update.
* remote-vx.c (vx_create_inferior): Update.
* mips-tdep.c (heuristic_proc_start): Update.
