Currently whenever we bind a unix domain socket we first try to unlink it. This is to avoid a scenario where we crashed and left the old file around (because bind can't succeed if a file is already there).

Rather than performing that flow, if instead we:

if (bind()) {
    // we're bound, done!
    return;
}

if (non_blocking_connect_succeeds_or_fails_with_eagain()) {
    // someone else is listening, go away!
    fassert("address in use");
}

// there's a file there, but no one listening.  I.e. the old path
unlink();
bind();
return;

Then we can avoid trampling on a running mongod and instead fail the process that's starting up