No functional exit function. #140

$ ./gpython 
Python 3.4.0 (none, unknown)
[Gpython dev]
- os/arch: linux/amd64
- go version: go1.16.6
>>> exit()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    FIXME line of source goes here
NameError: "name 'exit' is not defined"
>>> import sys
>>> sys.exit()
<SystemExit instance at 0xc000208180>
>>> quit()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    FIXME line of source goes here
NameError: "name 'quit' is not defined"