Add <sys/atomic.h>, defining the API for atomic operations and memory
barriers.
Man pages for the atomic ops API.
Fix a typo.
Add a header file that contains convenience macros for implementing
other types of atomic operations in terms of atomic_cas_32() (including
sub-32-bit operations).

XXX Probably does not work on big-endian yet.
Add generic atomic_add routines built on atomic_cas.
Make this work for big-endian with a trick provided by Matt Thomas.
Add generic atomic_and routines built on atomic_cas.
Add generic atomic_or routines built on atomic_cas.
Add generic atomic_cas_8() and atomic_cas_16() routines built on
atomic_cas_32().
Add generic atomic_inc and atomic_dec routines built on atomic_add.
Add generic atomic_swap routines built on atomic_cas.
Namespace protection for atomic operations.
Add atomic op implementations for x86.
Fix copy-and-pasto.
Add the necessary aliasa for _atomic_cas_32().
Give the namespace treatment to _atomic_cas_32_486(), too.
First draft implementation of atomic ops for amd64
Fix the end-of-function padding so both versions of cas-32 end up the
same size.
Add build glue for i386 atomic ops.
Hot-patch the atomic ops that need it on x86 (32-bit).