Add PATH_MAX and FILENAME_MAX for Windows and Redox

[oech3]

I noticed that thost consts are missing from libc when I tried to port uutils/pathchk.
Would you consider adding them?

[ChrisDenton]

FILENAME_MAX could be set to 260 as it is in stdio.h on msvc targets. But note that this is the length in UTF-16 code units, not bytes.

PATH_MAX doesn't exist on Windows msvc targets so I'm less sure about adding it. The nearest equivalent is MAX_PATH, which is 260 (again in UTF-16 code units). But that's a "soft" maximum. The actual OS limit is 32767.

[oech3]

Oh UTF-16... Supporting pathchk would be bit complex...