Applied patch from Dan Egnor improving the base64 code.

* base64.h (BASE64_ENCODE_LENGTH): New macro.
(struct base64_ctx): New context struct, for decoding.
(BASE64_DECODE_LENGTH): New macro.
* base64.c (base64_decode_init): New function.
(base64_decode_update): New function, replacing base64_decode.
Takes a struct base64_ctx argument.
* nettle-meta.h: Updated nettle_armor, and related typedefs and
macros.
* configure.in: Use AC_PREREQ(2.50).

Rev: src/nettle/nettle-meta.h:1.4