htslib: fix mtime_local sub-second precision on POSIX

mtime_local() returns milliseconds since the epoch, but the POSIX
branch divided tv_usec (microseconds) by 1000000 instead of 1000,
dropping the entire millisecond term. The clock only advanced at
whole-second boundaries, so every sub-second delta the callers compute
(request/connect timing, transfer-rate smoothing) read as zero. The
Windows ftime() branch was already correct.

Found while documenting the public API (#382).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

src/htslib.c

@@ -2580,8 +2580,8 @@ HTSEXT_API TStamp mtime_local(void) {
     assert(! "gettimeofday");
   }
 
-  return (TStamp) (((TStamp) tv.tv_sec * (TStamp) 1000)
-                   + ((TStamp) tv.tv_usec / (TStamp) 1000000));
+  return (TStamp) (((TStamp) tv.tv_sec * (TStamp) 1000) +
+                   ((TStamp) tv.tv_usec / (TStamp) 1000));
 #else
   struct timeb B;
   ftime(&B);