Factored out a lot of common code.

Factored out a lot of common code.

A new module is created: services.util which has a lot of helper classes
and functions to do common tasks of service handlers. Actual classes are:
Persistent, Restorable, ConfigWriter, ServiceHandler, InitdHandler and
TransactionalHandler (see documentation for details).

A call() function is provided as a simple wrapper for subprocess.call() to
execute simple commands. It can raise and ExecutionError, as a wrapper for
subprocess.call exceptions or ReturnNot0Error if the returned value is not
0.

All actual service handlers were updated.