Add support for HTTP over unix sockets as transport to deliver events

[nilmerg]

In addition to HTTP(S) over TCP, sources should be able to establish contact with Icinga Notifications via a Unix socket. It is assumed that the respective source is running on the same system. Therefore, sources do not need to authenticate, but only identify themselves. They can specify only the username; a password or certificate is not required.

We will generally recommend this architectural approach for our own sources.