Expose as fd + opts can set tcp_user_timeout

[fulara]

TCP timeouts are hard.

I have observed that in one specific scenario, namely when target endpoint goes down in aws land, application connecting to db hangs for around 15 minutes, even with setting the connect_timeout, I don't want to set read_timeout as that may have undesirable effect on long executing queries

Now, my knowledge in tcp is very accidental, but the TCP_USER_TIMEOUT seems to achieve the desired behavior.

I have added 2 commits:

1. impl as_raw_fd(), this way I could have modified the user_timeout without having to hack the library
2. Since I am here, add support in Opts to handle tcp_user_timeout via ops.

I have not tested whether this builds on non linux platform.