sugar-jhbuild shell should change the prompt

[manuq]

This will prevent confusions while working inside a Sugar shell. The PS1 variable should change to identify it.

Proposal: prefixing PS1 with '(sugar)', like:

[humitos] [~/sugar]$ PS1=(sugar)$PS1
(sugar)[humitos] [~/sugar]$

Besides, we should save the old PS1 to be able to restore it when the
user closes the "sugar" session.

[humitos]

I was taking a look at this, and it's no trivial because that variable is set by ~/.bashrc or /etc/bash.bashrc. So, if we set this variable in the sugar config script; it will be override when that script was called (after os.execlp call).

[humitos]

The patch attached only works if the user doesn't have set the PS1 variable but that is not so common.

That patch shows where we should set the PS1 environment variable, at least :)