[cmap] improve format 12 subtable error messages
[ot-sanitise] make writing output optional

Output file has to explicitly given now, no more writing to stdout, and
if no output file is specified, it will just check the font and no data
will be output, so that it can be used as simple font checker.
[CFF] recover code the dumps CharString operators

… and hide it behind #ifdef, reverts part of
7eb5a0433add5d8ea2b5288836d7add4cf61d0e5.
[CFF] don’t reject fonts using dotsection operator

It is deprecated operator but harmless nonetheless and font rastrizers
will be ignoring it anyway.

Fixes #6