function so session setup can modify the user environment as needed.
For PAM authentication, merge the PAM environment with the user
environment at init_session time. We no longer need to swap in the
user_env for environ during session init, nor do we need to disable
the env hooks at init_session time.
hooks for getenv, putenv, setenv and unsetenv. This makes it
possible for the plugin to trap changes to the environment made by
authentication methods such as PAM or BSD auth so that such changes
are reflected in the environment passed back to sudo for execve().
