Custom implementation of Base64 decoding function.

Compared to the previous implementation based on OpenSSL, the new implementation
has these advantages:

 1. Strict and reliable detection of invalid strings, including strings with
    less than 4 bytes of garbage at the end;

 2. Allows to use Base64 strings without '=' padding.

src/test/nxt_tests.c

@@ -162,6 +162,10 @@ main(int argc, char **argv)
         return 1;
     }
 
+    if (nxt_base64_test(thr) != NXT_OK) {
+        return 1;
+    }
+
 #if (NXT_HAVE_CLONE_NEWUSER)
     if (nxt_clone_creds_test(thr) != NXT_OK) {
         return 1;