Improve sysctl handling
The sysctl handling needs some help. Hard coding values in configurations is problematic, in cases where the default value needs to change or the sysctl is removed entirely. Changing the <value> in the configs to <value>default</value> rather than hard coding is probably the least intrusive solution. Then if it's set to default, we set the value that is defined as default elsewhere for that version. If the user wants to specify the value, they can and it will never change. Otherwise config upgrade code could stomp on people's intended configurations.
Also need to verify no sysctls in the default config.xml are missing from the config upgrade code.
Add lookup table for sysctl tunable (sysctl.inc). Make config.xml values default to value 'default' Ticket #71