Fix PosixTimeval.ToString() when microseconds is < 100000

Bug would cause PosixTimeval.ToString() with a seconds of 123 and a microseconds of 1234 to return "123.1234"
but this should be returning "123.001234" (note the leading zeros).

Change from StringBuilder.Append() to StringBuilder.AppendFormat() and add a format that will add leading '0's
until there are 6 digits of microseconds.

Bug reported by github user bucweat.

SharpPcap/PosixTimeval.cs

@@ -268,7 +268,7 @@ public override System.String ToString()
             System.Text.StringBuilder sb = new System.Text.StringBuilder();
             sb.Append(Seconds);
             sb.Append('.');
-            sb.Append(MicroSeconds);
+            sb.AppendFormat("{0:000000}",MicroSeconds);
             sb.Append('s');
 
             return sb.ToString();

Test/PosixTimevalTest.cs

@@ -52,5 +52,14 @@ public void OperatorTest ()
             Assert.IsTrue(p1 <= p2, "p1 <= p2");
             Assert.IsTrue(p2 >= p1, "p2 >= p1");
         }
+
+        // Test string formatting output
+        [Test]
+        public void ToStringTest()
+        {
+            var p1 = new PosixTimeval(123, 12345);
+
+            Assert.AreEqual("123.012345s", p1.ToString());
+        }
     }
 }