Add support for service running status and restoring at startup.

Now ServiceHandler stores a persistent status of the service, to know if
the service should be started at startup (or reloaded when reload command
is called). To do that it inherits from Restorable, so restarting and
committing is now not done at _restore() any more. InetdHandler now
inherits from ServiceHandler to take advantage of its features.

ServiceHandler now support callables _service_{start,stop,restart,reload}
attributes to ease the customization.

PppHandler is a special case where each connection has it own running
status. All start, stop, restart and reload takes the name argument as
optional. If a name is not provided, all connections are processed. On
restart, all connections are (re)started, on reload (and at startup), only the previous
running connectinos are restarted.

ConfigWriter's _write_config() now can return True to indicate that commit
command shouldn't reload the service.

VrrpHandler now uses python's kill to stop the service.