manager: add Reload() D-Bus command

Add new Reload D-Bus command to reload NetworkManager configuration.

For now, this is like sending SIGHUP to the process. There are several
advantages here:

  - it is guarded via PolicyKit authentication while signals
    can only be sent by root.

  - the user can wait for the reload to be complete instead of sending
    an asynchronous signal. For now, we operation completes after
    nm_config_reload() returns, but later we could delay the response
    further until specific parts are fully reloaded.

  - SIGHUP reloads everything including re-reading configuration from
    disk while SIGUSR1 reloads just certain parts such as writing out DNS
    configuration anew.
    Now, the Reload command has a flags argument which is more granular
    in selecting parts which are to be reloaded. For example, via
    signals the user can:

      1) send SIGUSR1: this writes out the DNS configuration to
         resolv.conf and possibly reloads other parts without
         re-reading configuration and without restarting the DNS plugin.
      2) send SIGHUP: this reloads configuration from disk,
         writes out resolv.conf and restarts the DNS plugin.

    There is no way, to only restart the DNS plugin without also reloading
    everything else.

libnm-core/nm-errors.h

@@ -185,6 +185,7 @@ GQuark nm_device_error_quark (void);
  *   enabled/disabled.
  * @NM_MANAGER_ERROR_UNKNOWN_LOG_LEVEL: Unknown log level in SetLogging
  * @NM_MANAGER_ERROR_UNKNOWN_LOG_DOMAIN: Unknown log domain in SetLogging
+ * @NM_MANAGER_ERROR_INVALID_ARGUMENTS: Invalid arguments for D-Bus request
  *
  * Errors related to the main "network management" interface of NetworkManager.
  * These may be returned from #NMClient methods that invoke D-Bus operations on
@@ -204,6 +205,7 @@ typedef enum {
 	NM_MANAGER_ERROR_ALREADY_ENABLED_OR_DISABLED, /*< nick=AlreadyEnabledOrDisabled >*/
 	NM_MANAGER_ERROR_UNKNOWN_LOG_LEVEL,           /*< nick=UnknownLogLevel >*/
 	NM_MANAGER_ERROR_UNKNOWN_LOG_DOMAIN,          /*< nick=UnknownLogDomain >*/
+	NM_MANAGER_ERROR_INVALID_ARGUMENTS,           /*< nick=InvalidArguments >*/
 } NMManagerError;
 
 GQuark nm_manager_error_quark (void);