sys/stdarg.h and expect compiler to provide proper builtins, defaulting
to the GCC interface. lint still has a special fallback.
Reduce abuse of _BSD_VA_LIST_ by defining __va_list by default and
derive va_list as required by standards.
Remove offset argument, we should now find an HFS+ volume in any
of its standard places.
Based on work from and test image provided by Pelle Johansson.
