If the list of tracked file descriptors is modified to contain unwanted
values (e.g. when a parent process closes or redirects stderr), this
makes it possible to set the tracked file descriptors back to a given
list.
This problem occurred with startx which somehow closes or redirects
stderr. As COLORED_STDERR_FDS was set in the shell running startx, this
disabled coloring stderr for all processes started in the X11 session.
This is no longer an issue. As COLORED_STDERR_FDS is set in the shell
configuration file, it automatically sets the list to the correct value
when a shell is opened in the terminal emulator once X11 is running.
Prevents programs from removing our environment variables during the
exec.
Also reduces unnecessary calls to update_environment() by calling it
only once before the exec.
