include syslog server to listen on udp/socket

Would be great to include a small go based syslog server library ( https://github.com/ziutek/syslog ) and allow via options to have logspout listen on udp syslog or unix socket.

this would allow pointing syslog in a container to logspout via udp or a volume mounted syslog socket. This would help apps that only support syslog logging run better in a container.

The issue names no files, tests, or entry points. Start by reviewing logspout's current log input and configuration paths, then assess the referenced ziutek/syslog library and the UDP and Unix-socket requirements; done means containerized applications can send syslog logs through either supported option.

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