Adding "conf.d" configuration dir support.

Hi Guus,

It's a copy of pull request #12 to avoid incompatibility issues between versions 1.0 and 1.1.

Any file matching the pattern /etc/tinc/$NETNAME/conf.d/*.conf will be
parsed after the tinc.conf file.

Differences with pull requests #12 are:
- I'm not using logger anymore (as you removed it in your last commit)
- I'm using the SLASH definition