Assign ss_len if it exists.

[Berrysoft]

Fixes #468

I don't remove len field because:

• socklen_t is larger than u8.
• It will introduce more platform-specific code.
• Usual socket methods (at least on macOS) don't check ss_len. We still need another field to pass in and out.

[Thomasdezeeuw]

I don't really know if we need it in another places as well. For example in calls to accept? Does the OS set ss_len?

[Berrysoft]

AFAIK, macos sets ss_len to the correct value in all syscalls, e.g., accept, getpeername, getsockname.

[Thomasdezeeuw]

Thanks @Berrysoft