KRT: Allow to learn routes with RTPROT_KERNEL

The Kernel protocol, even with the option 'learn' enabled, ignores
direct routes created by the OS kernel (on Linux these are routes
with rtm_protocol == RTPROT_KERNEL).

Implement optional behavior where both OS kernel and third-party routes
are learned, it can be enabled by 'learn all' option.

Minor changes by committer.