Avoid need for base64_init and extra table

Avoid need for base64_init and extra table

Simplify the code by assuming C99 initializers.
* src/list.c (base_64_digits): Remove.
(base64_map): Now a constant.  Now has its (old value + 1) % 65,
as that’s the only easy portable way to do it with a static
initializer (even on platforms where CHAR_BIT != 8); all uses changed.
(base64_init): Remove; only use removed.
(from_header): Adjust to new values in base64_map.

* src/list.c (base_64_digits): Remove; no longer needed.
(base64_map): Now const, initialized statically, and with
invalid entries being 0 not 64, and with valid entries
being 1 greater than before.