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.h

@@ -64,6 +64,7 @@ nxt_int_t nxt_malloc_test(nxt_thread_t *thr);
 nxt_int_t nxt_utf8_test(nxt_thread_t *thr);
 nxt_int_t nxt_http_parse_test(nxt_thread_t *thr);
 nxt_int_t nxt_strverscmp_test(nxt_thread_t *thr);
+nxt_int_t nxt_base64_test(nxt_thread_t *thr);
 nxt_int_t nxt_clone_creds_test(nxt_thread_t *thr);