url probability bug fix and random number use update

[Val S. <va...@nv...>]

Hi Robert,
   There is a major bug in the curl-loader regarding use of the FETCH_PROBABILITY tag.  In loader_fsm.c, svn rev 574, line 1359 is

cctx->url_curr_index =  (size_t) url_next;

The url index is updated but not the url pointer.  The result of this bug is that all urls are selected regardless of the probability somewhat uniformly. 

While debugging this I also reworked the use of probability in the curl-loader to produce better pseudo-random numbers and also added a tag RANDOM_SEED that lets you control the seed.  I am attaching the patch against svn rev 574.  To use it

cd curl-loader
patch -p3 <curl-loader.patch.574+vs6

I also noticed that the tag FETCH_PROBAILITY_ONCE is not used, but will let you decide whether it's worth to keep.

Regards,
/Val