subsystem: Prevent endless loop

subsystem: Prevent endless loop

If a user has home directory "/" and login shell "*" then login and su
enter an endless loop by constantly switching to the next subsystem.

This could also be achieved with a layered approach so just checking
for "/" as home directory is not enough to protect against such a
misconfiguration.

Just break the loop if it progressed too far. I doubt that this has
negative impact on any real setup.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>