Clarifies system() source usage on Linux

[eldarnash]

Fixes syslog-ng/syslog-ng#4112

[bazsi]

The expansion of the system source is not perfect, but I am not sure we need to specify how that is formatted exactly.

The /dev/kmsg is used this way:

file("/dev/kmsg" program-override("kernel") flags(kernel) format("linux-kmsg") keep-timestamp(no));

The /proc/kmsg is used this way:

file("/proc/kmsg" program-override("kernel") flags(kernel) keep-timestamp(no));