Add server block option 'dynamicLookup', implementing dynamicLookupCommand+radsec.dynsrv.sh

The server block option dynamicLookupCommand makes radsecproxy invoke an executable and treat its output as part of the configuration. This is slightly scary from a security point of view.

We should add an option dynamicLookup which does what tools/radsec-dynsrv.sh does, namely resolving SRV _radsec._tcp.REALM and puts the resulting names in a server block, but does it natively through the resolver.

It should probably take two arguments -- the name and type of the server block.