1.0.15.15: reset DF on x86 and x86-64 after every STD instead of when calling out

1.0.15.15: reset DF on x86 and x86-64 after every STD instead of when calling out

 * Both the old and the new behaviour should be correct re C-ABI, but
   this is much easier to audit. Thanks to Aurelien Jarno for the
   initial patch.

 * This should also fix the build on platforms with GCC 4.3 compiled
   libc signal handling functions, which assume DF to be cleared.

   ** NOTE ** If kernel/libc does not arrange for DF to be cleared
   before calling our handler and restore it afterwards, we can still
   be broken by asynch signals on post 4.3 GCC libcs -- but this Is
   Not Our Fault, We Think.

 * Test-case suggested by Alistair Bridgewater for possible related
   default-unknown-values problems on SunOS.