unix: handle the optional IOV_MAX
unix: handle the optional IOV_MAX

IOV_MAX is optional in POSIX [1], so it is not defined if the OS does
not specify an actual limit. Hence, make lwt_unix_iov_max simply return
None, as it is handled already by the callers.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html

c-cube/lwt,ocsigen/lwt,c-cube/lwt,ocsigen/lwt,ocsigen/lwt,c-cube/lwt