hexdecoct: implicitly parse URL-safe base64 format, too

JSON-I (RFC 7493) suggests to use the URL safe base64 alphabet, rather
than the regular one when encoding binary data in JSON strings. We
generally uses the regular alphabet though.

Let's be tolerant in what we parse however: simply accept both formats
when we parse base64.

This does nothing about base64 generation though, only about parsing.

src/basic/hexdecoct.c

@@ -553,12 +553,12 @@ int unbase64char(char c) {
 
         offset += '9' - '0' + 1;
 
-        if (c == '+')
+        if (IN_SET(c, '+', '-')) /* Support both the regular and the URL safe character set (see above) */
                 return offset;
 
         offset++;
 
-        if (c == '/')
+        if (IN_SET(c, '/', '_')) /* ditto */
                 return offset;
 
         return -EINVAL;