[pulseaudio-discuss] Allow adding modargs from UCM

[arun at accosted.net]

I found this necessary on some hardware -- you might want to be able to
provide PulseAudio-specific flags on some hardware in UCM.

For example, ignore_dB might be needed if we have bad decibel data from
the driver. Similarly, I've come across cases where mmap support was
quietly broken, and being able to explicitly disable it was useful. Both
these cases should be fixed in drivers, but having this mechanism allows
us to not have to wait for that to happen during initial enablement.

Patch 1 adds API to append values to an existing set of modargs
Patch 2 adds a custom value in UCM that we can use to pass modargs to PA

-- Arun