[Ada] Ensure Ctrl-C is not emited on terminated processes

[Ada] Ensure Ctrl-C is not emited on terminated processes

Due to the reuse policy of PID on Windows. Sending a Ctrl-C to a dead
process might result in a Ctrl-C sent to the wrong process. The check is
also implemented on Unix platforms and avoid unecessary waits.

2019-07-22  Nicolas Roche  <roche@adacore.com>

gcc/ada/

	* terminals.c (__gnat_tty_waitpid): Support both blocking and
	not blocking mode.
	* libgnat/g-exptty.ads (Is_Process_Running): New function.
	* libgnat/g-exptty.adb (Close): Don't try to interrupt/terminate
	a process if it is already dead.

From-SVN: r273672