Doc and Config fixups.

Be explicit about linux package names for those not familiar with
the platform. Also more explicit about steps such as editing file,
and testing the daemon.

Remove leading whitespace from commented-out config examples.
Set default parallel DNS back to 10, and suggest override of 100 in
config file. Enforce skipped DNS for autoconf IPs if unset (this is
a consequence of the way Dancer merges the two files - top level
config cannot be in both files).

Netdisco/share/config.yml

@@ -162,8 +162,8 @@ workers:
   sleep_time: 2
 
 dns:
-  max_outstanding: 250
   no: ['fe80::/64','169.254.0.0/16']
+  max_outstanding: 10
 
 #housekeeping:
 #  discoverall: