Timing: fix mbedtls_set_alarm(0) on Unix/POSIX

The POSIX/Unix implementation of mbedtls_set_alarm did not set the
mbedtls_timing_alarmed flag when called with 0, which was inconsistent
with what the documentation implied and with the Windows behavior.
diff --git a/ChangeLog b/ChangeLog
index b3d4d51..bfba279 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -9,6 +9,7 @@
    * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.
      Found by projectgus and jethrogb, #836.
    * Fix usage help in ssl_server2 example. Found and fixed by Bei Lin.
+   * Fix mbedtls_timing_alarm(0) on Unix.
 
 = mbed TLS 2.6.0 branch released 2017-08-10