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.
to the target user immediately before setting resource limits, which
is after the monitor process has forked (so it has the old value).
Also, if the user did not authenticate, there is no pamh in the
monitor so we need to init pam here too. This means we end up
calling pam_start() twice, which should be fixed, but at least the
session is always properly closed now.
