Fixed signal races in shadow tools.

Some of the supplied tools use functions which are not signal-safe.

Most of the times it's exit() vs. _exit().

In other times it's how the standard output or standard error is
handled. FILE-related functions shall be avoided, therefore I replaced
them with write().

Also there is no need to call closelog(). At worst, it allows to
trigger a deadlock by issuing different signal types at bad timings.
But as these fixes are about race conditions, expect bad timings in
general for these bugs to be triggered. :)

src/sulogin.c

@@ -70,7 +70,7 @@ static RETSIGTYPE catch_signals (int);
 
 static RETSIGTYPE catch_signals (unused int sig)
 {
-	exit (1);
+	_exit (1);
 }
 
 /*