Add a config file option for controlling the enabled feature sources,
aimed at replacing the --sources command line flag which is now marked
as deprecated. The command line flag takes precedence over the config
file option.
Also, adds new method WaitForReady() into NfdMaster.
In practice, this quite widely tests nfd-master, too, as the tests
create an instance of NfdMaster and verify that the communication
between master and worker works.
Move most of the code under cmd/nfd-master and cmd/nfd-worker into new
packages pkg/nfd-master and pk/nfd-worker, respectively. Makes extending
unit tests to "main" functions easier.
