Implement Exit syscall

[topnax]

Specification:

Exit,		//ukonci proces/vlakno
                // IN: cx je exit code

//Navratove kody OS
enum class NOS_Error : uint16_t {
	Success = 0,				//vse v poradku
	Unknown_Filesystem,
	Invalid_Argument,			//neplatna kombinace vstupnich argumentu
	File_Not_Found,				//soubor  nenalezen
	Directory_Not_Empty,		//adresar neni prazdny (a napr. proto nesel smazat)
	Not_Enough_Disk_Space,
	Out_Of_Memory,	
	Permission_Denied,
	IO_Error,
	Unknown_Error = static_cast<uint16_t>(-1)		//doposud neznama chyba		
};

Exit(0) should be implicitly called whenever a cloned program ends.