Fix [info nameofexecutable] after [cd]

If argv0 is a relative path, [info nameofexecutable] returned
the wrong result after changing directory.
So calculate and stash the result during init.
Also move internal $jim_argv0 into namespace jim as $jim::argv0

Signed-off-by: Steve Bennett <steveb@workware.net.au>
initjimsh: jim::argv0 is not always available

Signed-off-by: Steve Bennett <steveb@workware.net.au>