Hardware flow control was discussed very recently - just scroll down the subject list a little.

Why theese control lines are omitted

Well, the 8051 is a cheap and simple microcontroller - you can't have everything!
I don't think it was particularly designed for hi-speed async comms - which is where you'd need HW flow control.

I think that they can be implemented via software
You just need to check CTS, and not transmit unless it's set, and only set RTS when you're ready to receive.

You can get the RS232 spec from:
http://www.tiaonline.org/standards.../EIA-232-F

The question is, whether you can do it fast enough in software (it's usually done in the hardware by UARTs)

Have you looked at the Triscend E5 - you can implement UARTs in the on-chip configurable logic.

RX and TX alone are enough to estabilish a safe serial link?

What do you mean by "safe?"