basenc: tweak checks to use unsigned char

basenc: tweak checks to use unsigned char

This tends to generate better code, at least on x86-64,
because callers are just as fast and callees can avoid a conversion.
* src/basenc.c: The following renamings also change the arg type
from char to unsigned char.  All uses changed.
(isubase): Rename from isbase.
(isubase64url): Rename from isbase64url.
(isubase32hex): Rename from isbase32hex.
(isubase16): Rename from isbase16.
(isuz85): Rename from isz85.
(isubase2): Rename from isbase2.

2023-10-24  Paul Eggert  <eggert@cs.ucla.edu>

* src/basenc.c (struct base16_decode_context):
Simplify by storing -1 for missing nibbles.  All uses changed.