posix_openpt is using the same convention as the auto-generated functions in zsyscall_GOOS_GOARCH.go in the syscall package, where the function is the same name as the syscall name in lowercase without the SYS_ prefix.

As for fd being int, I've looked at accept() and pipe() and etc. syscall wrappers as a guideline. All of which convert the raw uintptr result to int before the error check because the returned file descriptor is -1 to indicate an error value (in addition to the errno).