Eventscript function: change service_start into a function.

Eventscript function: change service_start into a function.

service_start is currently a variable.  This makes passing arguments
hard.  We change it to be a function and put default definitions into
the functions file.

We use a convention that if a service name argument is passed to a
redefined version of service_start() or service_stop() then it will
act unconditionally.  If no argument is passed then it can use
internal logic to decide if services should really be started.  This
is useful when a single eventscript handles multiple services.

This is a cherry-pick of ae38895 that needed to be reset mid-stream.
There is still some breakage following this commit.

Signed-off-by: Martin Schwenke <martin@meltin.net>
(This used to be ctdb commit 86e4aefed9fd1028660c98e3ea758c2b75ffc1d8)