base64 module: adjust API so it's compatible with gnulib's

base64 module: adjust API so it's compatible with gnulib's

* gl/lib/base64.c (base64_decode_ctx): If no context structure was passed in,
treat newlines as garbage (this is the historical behavior).  Formerly
base64_decode.
(base64_decode_alloc_ctx): Formerly base64_decode_alloc.
* gl/lib/base64.h (base64_decode): Macro for four-argument calls.
(base64_decode_alloc): Likewise.
* src/base64.c (do_decode): Call base64_decode_ctx instead of base64_decode.

Signed-off-by: Bo Borgerson <gigabo@gmail.com>