[System] Add missing ConfigureAwait (#5963)

This is a candidate fix for:
https://bugzilla.xamarin.com/show_bug.cgi?id=60317

The bug was introduced by 1b5e0f73316ee7b83c8947bc738015443fabd7af.

Preliminary verification
========================

If I make just this _one_ change to a "bad" commit of Mono from after
commit 1b5e0f7, rebuild System.dll in the `xammac` profile, and then
overwrite that file in the MonoBundle directory of a Xamarin.Mac test
app, then I restore the old "good" behavior.  (I started with
38da0b3b4996357a7472e3202c575c9111469721 as the "bad" version for my
tests.)

I believe this confirms that the cause of bug 60317 was the usual
deadlocking issue when awaiting Tasks on the UI thread.  As discussed on
https://blog.stephencleary.com/2012/07/dont-block-on-async-code.html, if
any Tasks awaited anywhere up the call stack are missing
`ConfigureAwait (false)`, then you can run into trouble on the UI
thread.

Additional little checks
========================

1. Do the other `await` statements in commit
1b5e0f73316ee7b83c8947bc738015443fabd7af all look OK?  Yes, it looks
like this was the only `await` statement that was missing
`ConfigureAwait (false)`:

```
$ git show 1b5e0f73316ee7b83c8947bc738015443fabd7af | grep await
+				await ProcessOperation (cancellationToken).ConfigureAwait (false);
+				var ret = await InnerRead (cancellationToken).ConfigureAwait (false);
+					await Parent.InnerWrite (RunSynchronously, cancellationToken);
+				var ret = await Parent.InnerRead (RunSynchronously, requestedSize, cancellationToken).ConfigureAwait (false);
+					result = await asyncRequest.StartOperation (CancellationToken.None).ConfigureAwait (false);
+				result = await asyncRequest.StartOperation (cancellationToken).ConfigureAwait (false);
+			var ret = await task.ConfigureAwait (false);
+			await task.ConfigureAwait (false);
```

2. Is it a concern that mcs/class/System did not contain any calls to
`ConfigureAwait ()` until commit 1b5e0f7:

```
$ git grep ConfigureAwait 1b5e0f73316ee7b83c8947bc738015443fabd7af^ -- mcs/class/System || echo 'None'
None
```

I do not think this is a concern.  For example,
System.Net.Http.HttpClient has been using `ConfigureAwait (false)` for
several years, so there is a precedent for it.

1 files changed, 1 insertions, 1 deletions

diff --git a/mcs/class/System/Mono.Net.Security/AsyncProtocolRequest.cs b/mcs/class/System/Mono.Net.Security/AsyncProtocolRequest.cs
index 8ed25908b0f..3406cae2c0f 100644
--- a/mcs/class/System/Mono.Net.Security/AsyncProtocolRequest.cs
+++ b/mcs/class/System/Mono.Net.Security/AsyncProtocolRequest.cs
@@ -227,7 +227,7 @@ namespace Mono.Net.Security
 				if (Interlocked.Exchange (ref WriteRequested, 0) != 0) {
 					// Flush the write queue.
 					Debug ("ProcessOperation - flushing write queue");
-					await Parent.InnerWrite (RunSynchronously, cancellationToken);
+					await Parent.InnerWrite (RunSynchronously, cancellationToken).ConfigureAwait (false);
 				}
 
 				Debug ("ProcessOperation done: {0} -> {1}", status, newStatus);