Add support for systemd/syslog priority levels to the console logger

[halter73]

In order to better support systemd app monitoring, we should add an option to the console logger to use systemd daemon logging prefixes as described here. This will allow better filtering of logs via journalctl. Basically we just need to prefix logs with <Numeric Log Number>. Based on the linked man page, we might also want to write to stderr instead of stdout, but I'm not sure if this is necessary.

See aspnet/KestrelHttpServer#2679 for original customer report.

[muratg]

Let's consider this for 3.0.0.

[gfoidl]

we might also want to write to stderr instead of stdout

For this see aspnet/Logging#661

[BrennanConroy]

Done via #1777