variable argument macros when possible.
* sigproc.cc: Throughout, use sigproc_printf rather than sip_printf.
* strace.cc (strace::prntf): Remove 'active' check, since callers are supposed
to ensure this.
(__system_printf): Remove. Subsumed by strace::prntf.
* winsup.h: Define "NEW_MACRO_VARARGS" to indicate when to use new macro
varargs capability.