fast treatment of many simultaneous transfers

both now provide the number of running handles back to the calling function.

set a private pointer added to the internal libcurl hash table for the
particular socket passed in to this function.

to be accurate on this.

macros.

and then runs a given command line.

to the 1024 file descriptor limit in the server...

up N idle and Z active connections in a controlled manner. This requires a
a HTTP server that supports the server end. I have a modified sws for this
(from the curl test suite) and I may commit the changes required for that
soonish.