better support for logging to syslog

[djugei]

this is kinda like #349 but bigger.

Have two format strings, the regular one is configured just as it currently is, and a second one that could be configured via .format_syslog in the builder.

The syslog formatter by default skips timestamps and such as those will be provided by the syslog.

env_logger then auto-detects if its running as a daemon, with an override available.
The most common service manager is systemd.
Detecting being run as systemd daemon is as simple as checking for the INVOCATION_ID environment variable.
Other service managers may require more complex detection.

I am willing to contribute the code, in fact i am working on it already, and will send a pr later.

[epage]

Please post your counter idea to #349 so we discuss all in one place how we should (or if we should) include syslog support, rather than splitting the conversation up by the different approaches that could be taken.