Patch by Fred Gansevles.

Patch by Fred Gansevles.

This patch solves 2 problems of the os module.
1) Bug ID #50 (case-mismatch wiht "environ.get(..,..)" and "del environ[..]")
2) os.environ.update (dict) doesn't propagate changes to the 'real'
   environment (i.e doesn't call putenv)

This patches also has minor changes specific for 1.6a
The string module isn't used anymore, instead the strings own methods are
used.