Remove panic in nat package on invalid hostport

Closes #14621

This one grew to be much more than I expected so here's the story... :-)
- when a bad port string (e.g. xxx80) is passed into container.create()
  via the API it wasn't being checked until we tried to start the container.
- While starting the container we trid to parse 'xxx80' in nat.Int()
  and would panic on the strconv.ParseUint().  We should (almost) never panic.
- In trying to remove the panic I decided to make it so that we, instead,
  checked the string during the NewPort() constructor.  This means that
  I had to change all casts from 'string' to 'Port' to use NewPort() instead.
  Which is a good thing anyway, people shouldn't assume they know the
  internal format of types like that, in general.
- This meant I had to go and add error checks on all calls to NewPort().
  To avoid changing the testcases too much I create newPortNoError() **JUST**
  for the testcase uses where we know the port string is ok.
- After all of that I then went back and added a check during container.create()
  to check the port string so we'll report the error as soon as we get the
  data.
- If, somehow, the bad string does get into the metadata we will generate
  an error during container.start() but I can't test for that because
  the container.create() catches it now.  But I did add a testcase for that.

Signed-off-by: Doug Davis <dug@us.ibm.com>

nat/nat_test.go

@@ -42,7 +42,11 @@ func TestParsePort(t *testing.T) {
 }
 
 func TestPort(t *testing.T) {
-	p := NewPort("tcp", "1234")
+	p, err := NewPort("tcp", "1234")
+
+	if err != nil {
+		t.Fatalf("tcp, 1234 had a parsing issue: %v", err)
+	}
 
 	if string(p) != "1234/tcp" {
 		t.Fatal("tcp, 1234 did not result in the string 1234/tcp")
@@ -59,6 +63,11 @@ func TestPort(t *testing.T) {
 	if p.Int() != 1234 {
 		t.Fatal("port int value was not 1234")
 	}
+
+	p, err = NewPort("tcp", "asd1234")
+	if err == nil {
+		t.Fatal("tcp, asd1234 was supposed to fail")
+	}
 }
 
 func TestSplitProtoPort(t *testing.T) {