Which of the GNU coreutils applications are included in the MacPorts coreutils package?
su has moved from coreutils to the utils-linux project since v8.18,
since it's more aligned with the linux specific routines in PAM etc.
http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commit;h=928dd737

For more portable and light weight switching of user credentials,
one can use gchroot --user=... /

I've amended wikipedia appropriately