mf: don't limit filename length with PATH_MAX

Use an arbitrary constant instead, which is as good as PATH_MAX.

This helps us to avoid having to think about pull request #523.

Also fix a case where a potentially signed char was passed to isspace().
io: make MP_PATH_MAX private to win32 code

The win32 code is the only thing which actually needs this (and it's
used to make emulation of UTF-8 filename APIs easier).