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

MFC r318582

Remove old spinlock_debug code from libc

This no longer seems useful. Remove it.

This was prompted by a "cast discards volatile qualifier" warning
in libthr when WARNS=6.

Sponsored by: Dell EMC

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

Fix typo. Note that this is mostly innocent, since libthr no longer
interposes the symbol.

Noted by: rpaulo
MFC after: 3 days

Properly interpose libc spinlocks, was missed in r276630. In
particular, stdio locking was affected.

Reported and tested by: "Matthew D. Fuller" <fullermd@over-yonder.net>
Sponsored by: The FreeBSD Foundation
MFC after: 3 days

Add prototypes for libc private interfaces.

While I'm there, apply __unused whenever appropriate.

Reviewed by: md5(1)

Remove 3rd clause, renumber, ok per email

Make _spinunlock() point to the spinunlock stub, not the
spinlock stub.

- Define a _spinunlock() function so that threading implementations may do
more complicated things than just setting the lock to 0.
- Implement stubs for this function in libc and the two threading libraries
that are currently in the tree.

Fix SCM ID's.

To be consistent, use the __weak_reference macro from <sys/cdefs.h>
instead of #pragma weak to create weak definitions. This macro is
improperly named, though, since a weak definition is not the same
thing as a weak reference.

Suggested by: bde

Remove _THREAD_SAFE and make libc thread-safe by default by
adding (weak definitions to) stubs for some of the pthread
functions. If the threads library is linked in, the real
pthread functions will pulled in.

Use the following convention for system calls wrapped by the
threads library:
__sys_foo - actual system call
_foo - weak definition to __sys_foo
foo - weak definition to __sys_foo

Change all libc uses of system calls wrapped by the threads
library from foo to _foo. In order to define the prototypes
for _foo(), we introduce namespace.h and un-namespace.h
(suggested by bde). All files that need to reference these
system calls, should include namespace.h before any standard
includes, then include un-namespace.h after the standard
includes and before any local includes. <db.h> is an exception
and shouldn't be included in between namespace.h and
un-namespace.h namespace.h will define foo to _foo, and
un-namespace.h will undefine foo.

Try to eliminate some of the recursive calls to MT-safe
functions in libc/stdio in preparation for adding a mutex
to FILE. We have recursive mutexes, but would like to avoid
using them if possible.

Remove uneeded includes of <errno.h> from a few files.

Add $FreeBSD$ to a few files in order to pass commitprep.

Approved by: -arch

$Id$ -> $FreeBSD$

Implement compile time debug mode for thread locks.

Stubs are required in libc so that it can be used with libpthread
(and kernel threads), but weak symbols and non-weak symbols of the
same name built into libc_r result in unpredictable linking.

Stub functions for thread locking with weak symbols so that they are
only linked when not linking an application against libc_r or libpthread.