pcm: multi plugin: detach the hw_ptr and appl_ptr from master_slave

pcm: multi plugin: detach the hw_ptr and appl_ptr from master_slave

Unfortunately, the master_slave buffer pointers are not always in sync with
the presented avail value and the higher layers (like write_areas) got
confused. Create own hw_ptr and appl_ptr.

This commit also tries to fix the hwsync and delay implementation (iterate
through all slaves).

The multi plugin was designed only for hardware which runs really in sync.
Anyway, users are trying to use this plugin for other purposes.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>