Sync gnulib base64 with coreutils base64.

Sync gnulib base64 with coreutils base64.
* lib/base64.c (base64_decode_ctx): If a decode context structure
was passed in use it to ignore newlines.  If a context structure
was _not_ passed in, continue to treat newlines as garbage (this
is the historical behavior).  Formerly base64_decode.
(base64_decode_alloc_ctx): Formerly base64_decode_alloc.  Now
takes a decode context structure.
* lib/base64.h (base64_decode): Macro for four-argument calls.
(base64_decode_alloc): Likewise.
* lib/base64.c (base64_decode_ctx): If a decode context structure
was passed in use it to ignore newlines.  If a context structure
was _not_ passed in, continue to treat newlines as garbage (this
is the historical behavior).  Formerly base64_decode.
(base64_decode_alloc_ctx): Formerly base64_decode_alloc.  Now
takes a decode context structure.
* lib/base64.h (base64_decode): Macro for four-argument calls.
(base64_decode_alloc): Likewise.