Input merger

This component merges the input events of multiple sources.

Example configuration:

<start name="input_merger">
	<resource name="RAM" quantum="1M" />
	<provides>
		<service name="Input" />
	</provides>
	<config>
		<input label="ps2" />
		<input label="usb_hid" />
	</config>
	<route>
		<service name="Input">
			<if-arg key="label" value="ps2" /> <child name="ps2_drv" />
		</service>
		<service name="Input">
			<if-arg key="label" value="usb_hid" /> <child name="usb_drv" />
		</service>
		<any-service> <parent /> <any-child /> </any-service>
	</route>
</start>

For each 'input' config node, the component opens an 'Input' session with the
configured label. This label is then evaluated by 'init' to route the session
request to a specific input source component.

Fixes #1259.