Add posix-y compatibility extension class over Mach::Semaphore.

spec/process_shared/semaphore_spec.rb

@@ -98,14 +98,13 @@ module ProcessShared
 
       it 'returns after waiting if another processes posts' do
         Semaphore.open(0) do |sem|
-          start = Time.now.to_f
-
           pid = fork do
             sleep 0.01
             sem.post
             Kernel.exit!
           end
 
+          start = Time.now.to_f
           sem.try_wait(0.1)
           (Time.now.to_f - start).must be_lt(0.1)