Cleanup mrt message handling. Remove the DUMP_XYZ() macros and replace

Cleanup mrt message handling. Remove the DUMP_XYZ() macros and replace
them with direct calls to for example ibuf_add_n16(). Further cleanup
the error handling and use goto fail in most places. Remove many of the
error messages and combine all the possible ibuf errors in one place.
For this remove most warnings from internal functions (also mark all
internal helper functions with static to make that more obvious).
There are still some cases where an error will result in to warnings but
those errors are unreachable in normal operations.
OK tb@