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.

auto/sources

@@ -170,6 +170,7 @@ NXT_TEST_SRCS=" \
     src/test/nxt_rbtree1_test.c \
     src/test/nxt_http_parse_test.c \
     src/test/nxt_strverscmp_test.c \
+    src/test/nxt_base64_test.c \
 "