util/async.c: Add async IO worker thread

Need this for IO4 USB emulation, might break it out and upstream it
to Capnhook at some point as well.

util/meson.build

@@ -10,6 +10,8 @@ util_lib = static_library(
         capnhook.get_variable('hook_dep'),
     ],
     sources : [
+        'async.c',
+        'async.h',
         'clock.c',
         'clock.h',
         'crc.c',