stdio.h: Fix function-type typedef style and use _types.h __ssize_t

I'm still not sure why only Pypy runs into the error with the function
typedefs. Fix it anyway.

Use __ssize_t instead of ssize_t for the types; it's possible for the size_t
type to not be visible if at the wrong POSIX_VISIBLE level.

A final (crossing my fingers) follow-up to r299456.

Sponsored by: EMC / Isilon Storage Division