Revise the logic for picking the start time for link certs

Since 0.2.4.11-alpha (in 0196647970a91d) we've tried to randomize
the start time to up to some time in the past.  But unfortunately we
allowed the start time to be in the future as well, which isn't
really legit.

The new behavior lets the start time be be up to
MAX(cert_lifetime-2days, 0) in the past, but never in the future.

Fixes bug 21420; bugfix on 0.2.4.11-alpha.

changes/bug21420

@@ -0,0 +1,3 @@
+  o Minor bugfixes (certificate expiration time):
+    - Avoid using link certificates that don't become valid till
+      some time in the future.  Fixes bug 21420; bugfix on 0.2.4.11-alpha

src/common/tortls.c

@@ -482,8 +482,22 @@ MOCK_IMPL(STATIC X509 *,
    * then we might pick a time where we're about to expire. Lastly, be
    * sure to start on a day boundary. */
   time_t now = time(NULL);
-  start_time = crypto_rand_time_range(now - cert_lifetime, now) + 2*24*3600;
-  start_time -= start_time % (24*3600);
+  /* Our certificate lifetime will be cert_lifetime no matter what, but if we
+   * start cert_lifetime in the past, we'll have 0 real lifetime.  instead we
+   * start up to (cert_lifetime - min_real_lifetime - start_granularity) in
+   * the past. */
+  const time_t min_real_lifetime = 24*3600;
+  const time_t start_granularity = 24*3600;
+  time_t earliest_start_time = now - cert_lifetime + min_real_lifetime
+    + start_granularity;
+  /* Don't actually start in the future! */
+  if (earliest_start_time >= now)
+    earliest_start_time = now - 1;
+  start_time = crypto_rand_time_range(earliest_start_time, now);
+  /* Round the start time back to the start of a day. */
+  start_time -= start_time % start_granularity;
+
+  end_time = start_time + cert_lifetime;
 
   tor_assert(rsa);
   tor_assert(cname);
@@ -517,7 +531,6 @@ MOCK_IMPL(STATIC X509 *,
 
   if (!X509_time_adj(X509_get_notBefore(x509),0,&start_time))
     goto error;
-  end_time = start_time + cert_lifetime;
   if (!X509_time_adj(X509_get_notAfter(x509),0,&end_time))
     goto error;
   if (!X509_set_pubkey(x509, pkey))