mprintf: overhaul and bugfixes

mprintf: overhaul and bugfixes

In a test case using lots of snprintf() calls using many commonly used
%-codes per call, this version is around 30% faster than previous
version.

It also fixes the #12561 bug which made it not behave correctly when
given unknown %-sequences. Fixing that flaw required a different take on
the problem, which resulted in the new two-arrays model.

lib557: extended - Verify the #12561 fix and test more printf features

unit1398: fix test: It used a <num>$ only for one argument, which is not
supported.

Fixes #12561
Closes #12563