[Ada] Add socket options to control keepalive on TCP connection

[Ada] Add socket options to control keepalive on TCP connection

gcc/ada/

	* libgnat/g-socket.ads (Option_Name): Add Keep_Alive_Count,
	Keep_Alive_Idle, and Keep_Alive_Interval items to enumeration.
	(Option_Type): Add Keep_Alive_Count, Keep_Alive_Idle, and
	Keep_Alive_Interval alternatives to the case of discriminated
	record.
	* libgnat/g-socket.adb (Options): Add Keep_Alive_Count,
	Keep_Alive_Idle, and Keep_Alive_Interval to items enumerator to
	OS constant converter.
	(Set_Socket_Option): Process Keep_Alive_Count, Keep_Alive_Idle,
	and Keep_Alive_Interval socket options.
	(Get_Socket_Option): Idem.