returns an alist of (string x string) as obtained from the
system environ.
* Added the primitive env:
(env "key") retrieves the env value associated with key
(env "key" "val") sets the env value assocuated with key to val
(env "key" "val" overwrite?) same as above but it does not
overwrite an existing value of overwrite? is #f.
Abdulaziz Ghuloum