vfs_oss: add mixing support

[cproc]

The OSS API supports "virtual mixing" where /dev/dsp can be opened multiple times and the audio signals get mixed together [1]. This feature would be useful for QtWebEngine, which can currently get stuck when multiple threads try to output audio.

[1] http://manuals.opensound.com/developer/open.html

[cproc]

Fixed by cddb300, 7413491 and d2df898.

Since each /dev/dsp "session" has its own context data (info and ioctl files), commit cddb300 adds a _context_id member to the Vfs::Vfs_handle class, which can be used to determine the corresponding path in the hidden context directory (/dev/.dsp/0/, /dev/.dsp/1/ ...). Setting the context id is optional and the libc only appends it to the ioctl path if it is valid.

[cproc]

The actual mixing is done by the record_play_mixer component. The VFS plugin just creates a new Audio instance for each open("/dev/dsp") with dedicated Record and Play sessions.