System: More work on making system functions pluggable

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.

src/tools/pipewire-cli.c

@@ -1312,7 +1312,7 @@ static bool parse(struct data *data, char *buf, size_t size, char **error)
 	return false;
 }
 
-static void do_input(void *data, int fd, enum spa_io mask)
+static void do_input(void *data, int fd, uint32_t mask)
 {
 	struct data *d = data;
 	char buf[4096], *error;