[PATCH] atomic.h fixes + testcase, misc fixes

[Roland McGrath]

I've put in these changes and your later patch.  I made some more tweaks,
though things are still all consistent.  I am not really sure what the best
semantics for all the macros with boolean results are.  It depends on the
usage and the cheapness on processors where it makes a difference.  So far
only atomic_decrement_and_test is actually in use in libc, and for that the
i486 definition's semantics make sense.