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

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

Remove 3rd clause, renumber, ok per email

Remove #include of spinlock.h from libc_private.h. Declare spinlocks as
struct _spinlock. Keep the typedef in for now; another set of changes
may come around to clean up consumers of spinlocks.

Requested by: bde

- 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.

Remove __P() usage.

$Id$ -> $FreeBSD$

Implement compile time debug support for spinlocks.

Simplify the atomic lock prototype, removing the lock value.

Delete the unlock prototypes that are not required.

In a threaded library, expect the lock field to be declared volatile,
so provide function prototypes that respect that, avoiding a gcc
warning that `volatile' is being thrown away.

Add a private (to libc, libc_r and libpthread) header file containing
prototypes for the spinlock functions that will be used for thread locks.
libc will have stubs declared with weak symbols. libpthread and libc_r
will have functions that really do something.