gcc/linux updates

Many updates to format strings, largely as a result of changing
various "long" variables to uint32_t.

Fixed the diskimg debug macros for gcc, which requires an extra
"##" to remove the "," when there are no arguments.  (Apparently
Visual Studio just strips this away for you.)

Stripped out a couple of dead variables spotted by gcc.  Return
the actual error in a couple of HFS file functions.