[core] defer li_rand_init() until first use

defer li_rand_init() until first use of li_rand_pseudo_bytes()

li_rand_init() is now deferred until first use so that installations
that do not use modules which use these routines do need to potentially
block at startup.  Current use by core lighttpd modules is in mod_auth
HTTP Digest auth and in mod_usertrack.  Deferring collection of random
data until first use may allow sufficient entropy to be collected by
kernel before first use, helping reduce or avoid situations in
low-entropy-generating embedded devices which might otherwise block
lighttpd for minutes at device startup.  Further discussion in
https://redmine.lighttpd.net/boards/2/topics/6981

src/rand.c

@@ -31,6 +31,15 @@
  * block, and are intended to be called only at startup in lighttpd, or
  * immediately after fork() to start lighttpd workers.
  *
+ * Update: li_rand_init() is now deferred until first use so that installations
+ * that do not use modules which use these routines do need to potentially block
+ * at startup.  Current use by core lighttpd modules is in mod_auth HTTP Digest
+ * auth and in mod_usertrack.  Deferring collection of random data until first
+ * use may allow sufficient entropy to be collected by kernel before first use,
+ * helping reduce or avoid situations in low-entropy-generating embedded devices
+ * which might otherwise block lighttpd for minutes at device startup.
+ * Further discussion in https://redmine.lighttpd.net/boards/2/topics/6981
+ *
  * Note: results from li_rand_pseudo_bytes() are not necessarily
  * cryptographically random and must not be used for purposes such
  * as key generation which require cryptographic randomness.
@@ -111,9 +120,10 @@ static int li_rand_device_bytes (unsigned char *buf, int num)
     return 0;
 }
 
+static int li_rand_inited;
 static unsigned short xsubi[3];
 
-void li_rand_reseed (void)
+static void li_rand_init (void)
 {
     /* (intended to be called at init and after fork() in order to re-seed PRNG
      *  so that forked children, grandchildren, etc do not share PRNG seed)
@@ -123,6 +133,7 @@ void li_rand_reseed (void)
      * https://github.com/libressl-portable/portable/commit/32d9eeeecf4e951e1566d5f4a42b36ea37b60f35
      */
     unsigned int u;
+    li_rand_inited = 1;
     if (1 == li_rand_device_bytes((unsigned char *)xsubi, (int)sizeof(xsubi))) {
         u = ((unsigned int)xsubi[0] << 16) | xsubi[1];
     }
@@ -149,6 +160,11 @@ void li_rand_reseed (void)
   #endif
 }
 
+void li_rand_reseed (void)
+{
+    if (li_rand_inited) li_rand_init();
+}
+
 int li_rand_pseudo_bytes (void)
 {
     /* randomness *is not* cryptographically strong */
@@ -159,6 +175,7 @@ int li_rand_pseudo_bytes (void)
     if (-1 != RAND_pseudo_bytes((unsigned char *)&i, sizeof(i))) return i;
   #endif
   #endif
+    if (!li_rand_inited) li_rand_init();
   #ifdef HAVE_ARC4RANDOM_BUF
     return (int)arc4random();
   #elif defined(HAVE_SRANDOM)