Move the epoll functions to the system functions and make the loop
use those. Use simple mask for events instead of enum.
Add the used system api in pw_loop.
Add System API to spa_support and use it where possible.
Pass the system API used in the realtime loops in spa_support as
well and use this in the realtime paths.
Improve bootstrapping, load only the log and cpu interfaces because
those can/need to be shared between instances. Let the core load
the other interfaces.
Add keys to configure the System and Loop implementations used in
pw_loop.
Make a new API to hide some the implementation of eventfd, timerfd
and signalfd along with clock and read/write/ioctl/close functions.
We would like to have plugins use the abstractions so that we
can switch them to something else when needed.