allow setting buffer sizes on server_socket

[travisdowns]

We add two options to set the recv and send (SO_RCVBUF, ...) buffer sizes on a listening socket (server_socket). This is mostly useful to propagate said sizes to all sockets returned by accept().

It is already possible to set the socket option directly on the connected socket after it returned by accept() but experimentally this results in a socket with the specified buffer size, but whose receive window will not be advertised to the client beyond the default (64K for current typical kernel defaults). So you get only some of the benefit of the larger buffer.

Setting the buffer size on the listening socket however, is mentioned as the correct approach in tcp(7) and does not suffer from the same limitation.

A test is included which checks that the mechanism, including the inheritance, works.