tree-wide: add global ascii_isdigit() + ascii_isalpha()

We now have a local implementation in string-util-fundamental.c, but
it's useful at a lot of other places, hence let's give it a more
expressive name and share it across the tree.

Follow-up for: 8d9156660d

src/basic/terminal-util.c

@@ -624,7 +624,7 @@ int vtnr_from_tty(const char *tty) {
         if (!startswith(tty, "tty") )
                 return -EINVAL;
 
-        if (tty[3] < '0' || tty[3] > '9')
+        if (!ascii_isdigit(tty[3]))
                 return -EINVAL;
 
         r = safe_atoi(tty+3, &i);