src/sulogin.c: Use a do-while loop

src/sulogin.c: Use a do-while loop

Clarify how this endless while(true) loop can be stopped by using a
boolean variable as condition and turn it into a do-while loop.

Suggested-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Samanta Navarro <ferivoz@riseup.net>