su -c currently calls setsid() to prevent TIOCSTI attacks as described in bug 
173008.

However such protection is not needed when running programs as root, only when 
running programs with lesser or incomparable privileges than the calling code.

The attached patch makes "su root -c command" not call setsid() and also gives 
a command-line option -C which does the same as -c but doesn't call setsid() 
(note that it's very important that the default option calls setsid() to deal 
with some proprietary software that uses "su -c" in system scripts).