system: directly use system shell as root shell

system: directly use system shell as root shell

Rather than through the /bin/sh symlink, because because invocating bash as
sh causes bash to mimic sh (see INVOCATION in bash(1)).

Signed-off-by: Philip Molloy <philip.a.molloy@gmail.com>
[yann.morin.1998@free.fr: use the system shell, not a new option,
 as suggested by Arnout]
[Peter: describe why this is done using the description from Philip's help
 text]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>