internal/task: implement PMutex

PMutex is a mutex when threading is possible, and a dummy mutex-like
object (that doesn't do anything) otherwise.

src/internal/task/pmutex-cooperative.go

@@ -0,0 +1,16 @@
+package task
+
+// PMutex is a real mutex on systems that can be either preemptive or threaded,
+// and a dummy lock on other (purely cooperative) systems.
+//
+// It is mainly useful for short operations that need a lock when threading may
+// be involved, but which do not need a lock with a purely cooperative
+// scheduler.
+type PMutex struct {
+}
+
+func (m *PMutex) Lock() {
+}
+
+func (m *PMutex) Unlock() {
+}