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/nxt_string.h

@@ -190,6 +190,8 @@ NXT_EXPORT uintptr_t nxt_encode_complex_uri(u_char *dst, u_char *src,
     size_t length);
 NXT_EXPORT nxt_bool_t nxt_is_complex_uri_encoded(u_char *s, size_t length);
 
+NXT_EXPORT ssize_t nxt_base64_decode(u_char *dst, u_char *src, size_t length);
+
 extern const uint8_t  nxt_hex2int[256];