fileio: improve API types

fileio: improve API types

Use size_t instead of uint32_t when specifying file sizes.  Update all
consumers up through the layers to use size_t when required.  These
changes should be safe, but the higher-levels will need to be updated
further to receive the intended benefits (i.e. large file support).

Add error checking for fileio_read and file_write.  Previously, all
errors were being silently ignored, so this change might cause some
problems for some people in some cases.  However, it gives us the chance
to handle any errors that do occur at higher-levels, rather than burying
our heads in the sand.