Function syscall_takes_fd_first 

pub fn syscall_takes_fd_first(sysno: Sysno) -> bool

True when sysno’s first argument is a file descriptor, so an anchor may restrict it with fd. The list is the fd-first syscalls a guest is likely to anchor on; an fd on any other syscall is refused when the spec is loaded rather than matched against an argument that is not a descriptor.