Call IRpcChannelBuffer_FreeBuffer to free the buffer and other resources 
allocated by IRpcChannelBuffer_GetBuffer and IRpcChannelBuffer_SendReceive.

This is to avoid calling HeapFree on random pointers when 
DeleteSecurityContext is called on a context that hasn't had a second 
InitializeSecurityContext called on it, which would otherwise intialise 
those pointers.

secur32: Try to retrive credentials by using the credential manager to retrieve credentials saved for the target server in InitializeContextHandleW, if possible.

secur32: If using cached credentials failed in InitializeSecurityContext then fail with SEC_E_NO_CREDENTIALS instead of carrying on with a blank password.

secur32: Move the detection of NULL credentials from AcquireCredentialsHandle to InitializeSecurityContext.

Only use cached credentials if the credentials were NULL. Don't pass a 
domain into ntlm_auth when using cached credentials as 
ntlm_auth/winbindd should be able to figure that out.

winex11.drv: Accomodate XGetWindowProperty returning an actual format of something other than 32 for the TARGETS property.

We shouldn't care what format the property was set with, as long as we
trust that the property is an atom type.