Allow passing in listen-mode sockets (SYN-402)

This issue has been migrated from [#1329](https://github.com/matrix-org/synapse/issues/1329).

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There's a number of situations in which it would be helpful if synapse could use existing listen-mode TCP sockets that are passed in as pre-opened FDs, instead of having it create them:

* sytest
* systemd
* any security-sensitive setup where synapse should be on port 80 or 443, and yet not run as root

By taking some commandline arguments with "fd" instead of "port", this could easily be managed; e.g.

python -m synapse.app.homeserver --listen-fd 3

(Imported from https://matrix.org/jira/browse/SYN-402)

(Reported by @leonerd)