Timing: fix set_alarm(0) on Unix/POSIX

The POSIX/Unix implementation of set_alarm did not set the
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 7903fc7..8b11c18 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -43,6 +43,7 @@
    * Fix word size check in in pk.c to not depend on MBEDTLS_HAVE_INT64.
    * Fix crash when calling mbedtls_ssl_cache_free() twice. Found by
      MilenkoMitrovic, #1104
+   * Fix mbedtls_timing_alarm(0) on Unix.
 
 Changes
    * Extend cert_write example program by options to set the CRT version