* IRegistryApi.cs: Modified GetValue to take RegistryValueOptions enum

* IRegistryApi.cs: Modified GetValue to take RegistryValueOptions enum
and removed obsolete return_default_value argument.
* RegistryKey.cs: Added missing ComVisible attribute on SetValue.
Use new GetValue method on IRegistryApi that takes RegistryValueOptions
enum. Added GetValue overload that takes RegistryValueOptions (2.0).
* Win32RegistryApi.cs: Modified GetValue to take RegistryValueOptions
enum, and removed obsolete returnDefaultValue argument. Added support
for ExpandString value type. Do no expand if DoNotExpand... is set.
* UnixRegistryApi.cs: Modified KeyHandler.GetValue to take
RegistryValueOptions enum, and do no expand environment variables in
an ExpandString if DoNotExpandEnvironmentNames is set. Modified
UnixRegistryApi.GetValue to take RegistryValueOptions enum, and
removed obsolete return_default_value argument.
* RegistryKeyTest.cs: Added test for expanding environment variables.
* corlib.dll.sources: Added RegistryValueOptions.cs.

svn path=/trunk/mcs/; revision=64076