build: only check for Linux fstatfs on Linux

This check incorrectly passed on Cygwin. While Cygwin has the
statfs.f_type field, it contains the volume's FileSystemAttributes,
which aren't useful for detecting network mounts.
stream_file: detect remote files on Cygwin

This is similar to native Windows, except it uses cygwin_conv_path.
Shared functionality has been moved to is_path_remote.