init: Use classes for parsing and clean up memory allocations

Create a Parser class that uses multiple SectionParser interfaces to
handle parsing the different sections of an init rc.

Create an ActionParser and ServiceParser that implement SectionParser
and parse the sections corresponding to Action and Service
classes.

Remove the legacy keyword structure and replace it with std::map's
that map keyword -> (minimum args, maximum args, function pointer) for
Commands and Service Options.

Create an ImportParser that implements SectionParser and handles the
import 'section'.

Clean up the unsafe memory handling of the Action class by using
std::unique_ptr.

Change-Id: Ic5ea5510cb956dbc3f78745a35096ca7d6da7085