add /etc/tinc/$NET/conf.d dir support

Hi Guus and happy christmas,

I love tinc, I'm using it in different projects and it really make my life (and some other people to) easier.

I'm using it with some debian packages for all the hosts in some VPN. I mostly use it to add new peers to the list of hosts so that we just have to add them in the "ConnectTo" parameter.
It would become a lot easier/better if the tinc.conf file could only contain the information specific to he current host ("Name") and if all the parameters like "ConnectTo", "Compression", "Device", etc. could be deployed by package in a different configuration file. If I want to change the compression level of the hosts I control in the VPN, I could add/modify a file /etc/tinc/myvpn/conf.d/compression.conf where I would put "Compression = <compLevel>".

In short terms using a "conf.d" dir would make our life easier. 

I tried to respect your coding style and updated the documentation. 

What do you think ?