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().
LOGIN_SETENV instead of rolling our own login.conf setenv support
since FreeBSD's login.conf has more than just setenv capabilities.
This requires us to swap the plugin-provided envp for the global
environ before calling setusercontext() and then stash the resulting
environ pointer back into the command details, which is kind of a
hack.
group_info and use it to store both, along with a count for each.
Cache group info on a per-user basis using getgrouplist() to get
the groups. We no longer need special to special case the user or
list user for user_in_group() and thus no longer need to reset the
groups list when listing another user.
