dhcpv4: server4: Add a simple logging implementation

[purpleidea]

This adds a simple implementation of the logging struct so that you don't need to implement the complex interface if you don't need it.

[purpleidea]

I think that the solution in https://github.com/insomniacslk/dhcp/pull/371/files was over-engineered, but in any case, I expect most people like myself just want simple logging. This patch adds a simple implementation so that you can easily "get logging" by passing in your own printf function straight away.

This is meant as an alternative to https://github.com/insomniacslk/dhcp/pull/373 which wasn't accepted.

Cheers!

[pmazzini]

Can you sign the commit?

[purpleidea]

@pmazzini You want to merge this now?