Fixes mono#16032

Switching to corefx File.Copy on unix changed the behavior where it opened a source and destination stream in order to pass
the safe handles to Interop.Sys.CopyFile.  This exposed a flaw in our sharing validation rules in MonoIO.Open, which
threw an exception in this scenario:

new FileStream("source.txt", FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.ReadWrite, 2048, FileOptions.DeleteOnClose);
new FileStream("source.txt", FileMode.Open, FileAccess.Read, FileShare.Read, 2048, FileOptions.None);

The change is intended to take into account all sharing scenarios.

mcs/class/corlib/Test/System.IO/FileStreamTest.cs

@@ -1650,6 +1650,30 @@ public void DeleteOnClose ()
 
 		}
 
+		// See https://github.com/mono/mono/issues/16032
+		[Test]
+		public void DeleteOnCloseWithFileCopy ()
+		{
+			string source = TempFolder + DSC + "source.txt";
+			string target = TempFolder + DSC + "target.txt";
+
+			File.WriteAllText (source, "text");
+			FileStream fs = new FileStream (source, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.ReadWrite, 2048,
+							FileOptions.DeleteOnClose);
+
+			Assert.AreEqual (true, File.Exists (source), "DCFC#1");
+
+			try {
+				File.Copy (fs.Name, target, true);
+			}
+			finally {
+				fs.Dispose();
+			}
+
+			Assert.AreEqual (false, File.Exists (source), "DCFC#2");
+			Assert.AreEqual (true, File.Exists (target), "DCFC#3");
+		}
+
 		[Test]
 		public void OpenCharDeviceRepeatedly ()
 		{

mono/metadata/w32file-unix.c

@@ -1874,16 +1874,15 @@ static gboolean share_allows_open (struct stat *statbuf, guint32 sharemode,
 			return(FALSE);
 		}
 
-		if (((file_existing_access & GENERIC_READ) &&
-		     !(sharemode & FILE_SHARE_READ)) ||
-		    ((file_existing_access & GENERIC_WRITE) &&
-		     !(sharemode & FILE_SHARE_WRITE))) {
-			/* New share mode doesn't match up */
-			mono_trace (G_LOG_LEVEL_DEBUG, MONO_TRACE_IO_LAYER_FILE, "%s: Access mode prevents open: requested share: 0x%" PRIx32 ", file has access: 0x%" PRIx32, __func__, sharemode, file_existing_access);
+		if (file_existing_share != sharemode &&
+				((sharemode | FILE_SHARE_READ) != sharemode) &&
+				((file_existing_share | sharemode) == file_existing_share)) {
+			/* Share mode is trying to be more strict */
+			mono_trace (G_LOG_LEVEL_DEBUG, MONO_TRACE_IO_LAYER_FILE, "%s: Can't restrict sharing mode: existing mode %d, requesting sharing mode: %d - the requested sharing mode is more strict", __func__, file_existing_share, sharemode);
 
 			file_share_release (*share_info);
 			*share_info = NULL;
-		
+
 			return(FALSE);
 		}
 	} else {