No consumers of readByte() were actually checking for EOF, so they would
all keep re-reading the same byte over and over again, potentially until the
process runs out of memory (ref #572).
* Add a va_list version which can be correctly used by ivprintf (fixes pycdas output)
* Use a `const char*` format parameter to avoid the need for clang warning workarounds
* Make better use of return values.
