jack: Allow to specify a client name as parameters

 The current jack client name contains the process id of the application
providing the audio samples. This leads to unpredictable jack client
names which makes handling of the connections by a controlling
application very hard.
 This modification now, allows to specify a client name as parameters
in the configuration file. The implementation is backward
compatible and simply adds a new configuration option to the plugin.

[reformatted and small refactoring by tiwai]

Signed-off-by: Valentin Corfu <valentinx.corfu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>