Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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