Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Revert r240060: (Partial)

__BEGIN_DECLS and __END_DECLS in cdefs.h take care of the
__cplusplus mangling issues so most of the definitions
were redundant.

In the few places where they were not redundant we should
use BSD style instead of the guards used upstream.

Reported by: Yuri Pankov

Fix RPC headers for C++

C++ mangling will cause trouble with variables like __rpc_xdr
in xdr.h so rename this to XDR.
While here add proper C++ guards to RPC headers.

PR: 137443
MFC after: 2 weeks

Add an implementation of the RPCSEC_GSS authentication protocol for RPC. This
is based on an old implementation from the University of Michigan with lots of
changes and fixes by me and the addition of a Solaris-compatible API.

Sponsored by: Isilon Systems
Reviewed by: alfred