[System*] Throw a PlatformNotSupported exception when using the managed networking...

[System*] Throw a PlatformNotSupported exception when using the managed networking stack on watchOS. (#3683)

* [System.Data] System.Data.SqlClient is not supported in watchOS.

System.Data.SqlClient is not supported in watchOS, because it depends on a
working managed networking stack, which we don't have on watchOS.

* [System] Throw PlatformNotSupportedExceptions in the networking stack for watchOS.

* [System] Don't reference/build Mono.Security.dll when building for watchOS.

This means we can't build Mono.Data.Tds.dll either, since it uses
Mono.Security.dll extensively. In any case Mono.Data.Tds.dll is useless
without a working networking stack, and since it's a Mono-only dll (i.e. not
part of netstandard or facades), just remove the entire assembly from the
watchOS profile.

System.Security.dll uses Mono.Security.dll as well, but the required classes
are available inside mscorlib.dll (as internal classes), so just open up
mscorlib internals to System.Security.dll.

* [System] Update tests after API change for watchOS.

* [System.Data] Update tests after API change in watchOS.

* Revert "Fix build break from 81fb20737f810def6dde88d87a1b2f23cdb736c3"

This reverts commit f4475ff01358f8490e7cd0878e8b39bed2a0def7.

* Fix missing trailing newline in monotouch_watch_System.dll.sources

It breaks the build on Linux if the newline is missing.