net: allow to set linger on TcpSocket

[fgaud]

For now, this is only allowed on TcpStream. This is a problem when one
want to disable lingering (i.e. set it to Duration(0, 0)). Without being
able to set it prior to the connect call, if the connect future is
dropped it would leave sockets in a TIME_WAIT state.

Motivation

We need to be able to disable lingering on the socket. Otherwise if we drop the connect call, close will cause the socket to move to TIME_WAIT which is a problem for us.

Solution

SO_LINGER is a socket option (https://man7.org/linux/man-pages/man7/socket.7.html), expose it as such similar to what is done for buffer sizes for example. I left the possibility to do that on the connected stream as well

Note that I reused the same comments than in TcpStream for consistency. I found that the last part [1] is a bit confusing but I thought that was better to keep that consistent.

[1] "If SO_LINGER is not specified, and the socket is closed, the system handles the call in a way that allows the process to continue as quickly as possible."

[Darksonn]

Seems good. Note that the method already exists on TcpStream, but adding it to TcpSocket as well is fine.