sd-dhcp-client: correct retransmission timeout to match RFC

This changes the retransmission timeout algorithm for requests
other than RENEW and REBIND. Previously, the retransmission timeout
started at 2 seconds, then doubling each retransmission up to a max
of 64 seconds. This is changed to match what RFC2131 section 4.1 describes,
which skips the initial 2 second timeout and starts with a 4 second timeout
instead. Note that -1 to +1 seconds of random 'fuzz' is added to each
timeout, in previous and current behavior.

This change is therefore slightly slower than the previous behavior in
attempting retransmissions when no server response is received, since the
first transmission times out in 4 seconds instead of 2.

Since TRANSIENT_FAILURE_ATTEMPTS is set to 3, the previous length of time
before a transient failure was reported back to systemd-networkd was
2 + 4 + 8 = 14 seconds, plus, on average, 3 seconds of random 'fuzz' for
a transient failure timeout between 11 and 17 seconds. Now, since the
first timeout starts at 4, the transient failure will be reported at
4 + 8 + 16 = 28 seconds, again plus 3 random seconds for a transient
failure timeout between 25 and 31 seconds.

Additionally, if MaxAttempts= is set, it will take slightly longer to
reach than with previous behavior.

src/libsystemd-network/sd-dhcp-client.c

@@ -1269,8 +1269,7 @@ static int client_timeout_resend(
                         goto error;
 
                 client->attempt++;
-                next_timeout = time_now + ((UINT64_C(1) << MIN(client->attempt, (uint64_t) 6)) - 1) * USEC_PER_SEC;
-                next_timeout += (random_u32() & 0x1fffff);
+                next_timeout = client_compute_request_timeout(time_now, client->attempt);
                 break;
 
         case DHCP_STATE_STOPPED: