Revert "Fix select timeout to be 50msec instead of 0.5msec" for acquire

The acquire system actually uses usec pulse intervals, so the
previous value was correct (500ms) whereas the new value is
now 5s.

It's a bit unfortunate that the two systems use different units
for pulse intervals, but probably not much we can do about it.

This partially reverts commit eaf21c2144fa8dc4be8581dc69cf88cb38e30ce2.

apt-pkg/acquire.h

@@ -281,7 +281,7 @@ class pkgAcquire
     *
     *  \return the result of the download.
     */
-   RunResult Run(int PulseInterval=50000000);
+   RunResult Run(int PulseInterval=500000);
 
    /** \brief Remove all items from this download process, terminate
     *  all download workers, and empty all queues.