0.9.18.6: Win32 get-internal-real-time improved

0.9.18.6: Win32 get-internal-real-time improved
 * The old version was accurate only to the second, new one gets
   milliseconds right.
 * Move unix/win32 implementation bodies of GET-INTERNAL-*-TIME to
   unix.lisp and win32.lisp (less conditionalization).
 * Move *GC-RUN-TIME* zeroing to GC-REINIT.
 * Initialize the system epoch to start time, not to time of first
   call to GET-INTERNAL-REAL-TIME, document system epoch in
   GET-INTERNAL-REAL-TIME.
 * Sort the stub function calls in src/runtime/win32.c nicely.