Implement base checkFn/prepFn/execFn model for daemons

This mirrors the code in the Python code base, and is required for
clean error reporting during startup.

This patch implements the basic infrastructure; the confd daemon is
not yet modified to take advantage of this, just the types are
adjusted.

Also, the pipe-based error reporting will go in a future patch, once
we can actually use that for reporting.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>