operations without ancillary data/fd

If there is some overhead related to the ancillary optional fd, would it be possible to have:
```
val Sendmsg.recv_no_fd: Unix.file_descr -> bytes -> int -> int -> int
val Sendmsg.send_no_fd: Unix.file_descr -> bytes -> int -> int -> int
```
Some users really have no use for this optional file_descr result (they never send one
and so never expect to receive one).
