unix: use socklen_t instead of size_t
unix: use socklen_t instead of size_t

It has been reported that it generates (otherwise harmless)
`-Wshorten-64-to-32` compiler warnings when building for iOS.

Fixes: https://github.com/libuv/libuv/issues/2714
PR-URL: https://github.com/libuv/libuv/pull/2716
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <a0f1490a20d0211c997b44bc357e1972deab8ae3@saghul.net>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso