Don't call bind() if the specified local address of a socket has port 0
and the "any" address. It will be bound automatically on connect() or
sendmsg().
On start, check if the SOCK_CLOEXEC and SOCK_NONBLOCK flags are
supported in the socket() call and use them instead of fcntl() in order
to reduce the number of system calls required to send a client request.
All networking code in chronyd (NTP server/client, signd client, cmdmon
server) assumes sending a message will not block, but only the signd
client actually checks for a write event and only the NTP server
requests a non-blocking socket. The cmdmon server and NTP client
(if using one socket for all servers) might be blocked.
chronyc doesn't need a non-blocking socket, but it is not expected to
block as it sends only one message at a time.
Prefer dropped messages over blocking in all cases. Remove the
SCK_FLAG_NONBLOCK flag and make all sockets non-blocking.
Add a new file implementing support for opening sockets, sending and
receiving messages with control messages (e.g. addresses, timestamps),
and related operations, which should be simpler to use than the system
functions and allow their features to be reused between different parts
of the chrony code.
It is based on the ntp_io.c and ntp_io_linux.c files. It will be used by
the NTP client/server, cmdmon server, client, and others.
