We should consider better API for Npth, when we support Windows 64-bit.

Perhaps, npth_t is better to be uintptr_t, even though it's 32-bit for the current implementation for 64-bit,
because the use of the type can be easily shared between POSIX system.

Also, for socket, see: T4655: Windows 64-bit: gnupg_fd_t, assuan_fd_t and int for fd in the API, and casts