Refactor: Use thread-locked goroutine for ptrace ops

Previously either the terminal client or the debugger service would
either lock main goroutine to a thread or provide a locked goroutine to
run _all_ DebuggedProcess functions in. This is unnecessary because only
ptrace functions need to be run from the same thread that originated the
PT_ATTACH request.

Here we use a specific thread-locked goroutine to service any ptrace
request. That goroutine is also responsible for the initial spawning /
attaching of the process, since it must be responsible for the PT_ATTACH
request.

proc/test/support.go

@@ -28,7 +28,7 @@ func BuildFixture(name string) Fixture {
 		return f
 	}
 	parent := ".."
-	fixturesDir := filepath.Join(parent, "_fixtures")
+	fixturesDir := "_fixtures"
 	for depth := 0; depth < 10; depth++ {
 		if _, err := os.Stat(fixturesDir); err == nil {
 			break