proc/native: fix FreeBSD backend (#3224)

- use PT_SUSPEND/PT_RESUME to control running threads in
  resume/stop/singleStep
- change manual stop signal from SIGTRAP to SIGSTOP to make manual stop
  handling simpler
- change (*nativeProcess).trapWaitInternal to suspend newly created
  threads when we are stepping a thread
- change (*nativeProcess).trapWaitInternal to handle some unhandled
  stop events
- remove misleading (*nativeProcess).waitFast which does not do
  anything different from the normal wait variant
- rewrite (*nativeProcess).stop to only set breakpoints for threads of
  which we have received SIGTRAP
- rewrite (*nativeThread).singleStep to actually execute a single
  instruction and to properly route signals

pkg/terminal/command_test.go

@@ -456,9 +456,6 @@ func TestScopePrefix(t *testing.T) {
 }
 
 func TestOnPrefix(t *testing.T) {
-	if runtime.GOOS == "freebsd" {
-		t.Skip("test is not valid on FreeBSD")
-	}
 	const prefix = "\ti: "
 	test.AllowRecording(t)
 	lenient := false
@@ -520,9 +517,6 @@ func TestNoVars(t *testing.T) {
 }
 
 func TestOnPrefixLocals(t *testing.T) {
-	if runtime.GOOS == "freebsd" {
-		t.Skip("test is not valid on FreeBSD")
-	}
 	const prefix = "\ti: "
 	test.AllowRecording(t)
 	withTestTerminal("goroutinestackprog", t, func(term *FakeTerminal) {