sd-bus: fix bus_poll() to handle EINTR properly

bus_ppoll() relies on ppoll(2) which can return EINTR when interrupted by a
signal. bus_ppoll() should be prepared to handle this case properly by
restarting the system call.

Hence let's make it call safe_ppoll_usec() instead since it does that
automatically for us.

This should address one of the cases that lead to the following error, which
might occur when shutting down the system:

 login[1489]: pam_systemd(login:session): Failed to release session: Interrupted system call

IOW this patch closes one possible root cause of issue systemd#18817.

src/libsystemd/sd-bus/sd-bus.c

@@ -3313,7 +3313,7 @@ static int bus_poll(sd_bus *bus, bool need_more, uint64_t timeout_usec) {
         if (timeout_usec != UINT64_MAX && (m == USEC_INFINITY || timeout_usec < m))
                 m = timeout_usec;
 
-        r = ppoll_usec(p, n, m);
+        r = safe_ppoll_usec(p, n, m);
         if (r <= 0)
                 return r;