Previously, all scripts had the same probability of being chosen when
multiple of them were specified via -b, -f, -N, -S.  With this commit,
-b and -f now search for an "@" in the script name and use the integer
found after it as the drawing probability for that script.

(One disadvantage is that if you have script whose names contain @, you
are now forced to specify "@1" at the end; otherwise the name's @ is
confused with a weight separator.  We don't expect many pgbench script
with @ in their names in the wild, so this shouldn't be too serious a
problem.)

While at it, rework the interface between addScript, process_file,
process_builtin, and findBuiltin.  It had gotten a bit out of hand with
recent commits.

Author: Fabien Coelho
Reviewed-By: Andres Freund, Robert Haas, Álvaro Herrera, Michaël Paquier
Discussion: http://www.postgresql.org/message-id/alpine.DEB.2.10.1603160721240.1666@sto