[Mono.Security.Interface] Cleanup and simplify MonoTlsProviderFactory.

[Mono.Security.Interface] Cleanup and simplify MonoTlsProviderFactory.

The "global" TLS Provider (returned by MonoTlsProviderFactory.GetProvider())
may only be modified at application startup (before any of the TLS / Certificate
code has been used).

* MonoTlsProviderFactory.HasProvider has been removed.

  This property used to initialize the TLS Subsystem without throwing any
  exceptions.  However, properties with side-effects is a bad design principle.

* Add MonoTlsProviderFactory.IsInitialized.

  Does not have any side-effects.

* Add MonoTlsProviderFactory.Initialize() and Initialize(string).

  May only be called at application startup and explicitly initializes the
  TLS Subsystem.

* Rename MonoTlsProviderFactory.GetDefaultProvider() into GetProvider()
  and removed MonoTlsProviderFactory.GetCurrentProvider().

  We do not distinguish between a "default" and a "current" provider anymore;
  there is only one "global" provider.

* Add MonoTlsProviderFactory.IsProviderSupported(string).

  Checks whether a specific TLS Provider is supported, without having side-effects.

* Make the old APIs [Obsolete] until products have been updated.

* Add some documentation to MonoTlsProviderFactory.

(cherry picked from commit 7c3ae83b31a2456d00daf2949706719e3e694d15)