general: remove mprintf calls, use pp_printf

We have not been using mprintf for ages now, and it's better for the
code to explicitly call pp_printf, so people is aware of it.

We might "#define printf pp_printf", but currently we'd better not.
Maybe when we turn this into a real operating system, next millennium...

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>