Support receiving socket file descriptors from systemd

#### Feature Description

Currently, consul supports notify systemd via `NOTIFY_SOCKET` when consul is ready, which allows other systemd services attempting to connect to consul to have consul as a dependency, but this doesn't help the case where there are services that don't declare consul as a systemd dependency (or are not systemd units at all) that are attempting to connect to consul while it is starting up.
Systemd has a solution to this, by specifying `ListenStream` in the service unit file. This informs systemd to start listening on the specified socket file descriptors before starting the service (and not reading/writing anything on the file descriptors), and then the service can get the file descriptor for the socket using [`sd_listen_fds`](https://www.freedesktop.org/software/systemd/man/sd_listen_fds.html#) (there can be multiple socket file descriptors that systemd opens for the service). Consul should implement support for listening on it's ports using this if it's available. Then users deploying Consul with systemd could write a systemd service unit file specifying for example `ListenStream=8500`, for example to have systemd start listening on port 8500 on behalf of consul before consul is completely ready.

#### Use Case(s)

During startup, after the consul process has been started executing but before it has opened and listened on it's sockets, there is a race condition where other services will attempt to connect and the connection will fail. This is especially problematic during reboot/restart scenarios where there can be many services starting simultaneously attempting to connect to consul. Implementing support for receiving socket file descriptors from systemd solves this, as usually in system startup scenarios systemd will start listening on all service listen streams very early in the boot cycle.