bindat/connectat syscalls

[Daurnimator]

Related thread (2012): https://marc.info/?l=linux-netdev&m=133897212713981&w=2

FreeBSD[1] chose signatures of:
    int bindat(int fd, int s, const struct sockaddr *addr, socklen_t addrlen);
    int connectat(int fd, int s, const struct sockaddr *name,
socklen_t namelen);

An alternate implementation could instead augment `sockaddr_un`.

Also, if we're going to add a new bind and connect() syscalls, we
should also add a flags argument.

(Please CC on reply)

[1] https://www.freebsd.org/cgi/man.cgi?query=bindat&sektion=2
    https://www.freebsd.org/cgi/man.cgi?query=connectat&sektion=2