[PATCH] x86_64: clean up ptrace single-stepping

Ported from i386 (originally from Linus)

clean up ptrace single-stepping, make PT_DTRACE exact.
  
  (This makes the naming of "DTRACE" purely historical, since
  on x86 it now means "single step in progress").

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>