MINOR: tools: preset the port of fd-based "sockets" to zero

MINOR: tools: preset the port of fd-based "sockets" to zero

Addresses made of a file descriptor store the file descriptor into the
address part of a sin_addr. Contrary to other address classes, there's
no way to figure later based on the FD if an initialization was done
(which is how logs initialize their FDs). The port part is currently
left with random data, so let's instead specifically set the port part
to zero when creating an FD, and let the code using it set whatever
info it needs there, typically an initialization state.