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_tls.h

@@ -98,8 +98,6 @@ extern const nxt_tls_lib_t        nxt_openssl_lib;
 void nxt_cdecl nxt_openssl_log_error(nxt_task_t *task, nxt_uint_t level,
     const char *fmt, ...);
 u_char *nxt_openssl_copy_error(u_char *p, u_char *end);
-nxt_int_t nxt_openssl_base64_decode(u_char *d, size_t dlen, const u_char *s,
-    size_t slen);
 #endif
 
 #if (NXT_HAVE_GNUTLS)