Remove pgbench's restriction on placement of -M switch.

Remove pgbench's restriction on placement of -M switch.

Previously the -M switch had to appear before any switch that directly
or indirectly specified a benchmarking script.  This was both confusing
and inadequately documented, as per gripe from Tatsuo Ishii.  We can
remove the restriction at the cost of making an extra pass over the
lists of SQL commands, which seems like a cheap price (the string scans
themselves likely cost much more).  The change is just to not extract
parameters from the SQL commands until we have finished parsing the
switches and know the final value of -M.

Per discussion, we'll treat this as a low-grade bug fix and sneak it
into v10, rather than holding it for v11.

Tom Lane, reviewed by Tatsuo Ishii and Fabien Coelho

Discussion: https://postgr.es/m/20170802.110328.1963639094551443169.t-ishii@sraoss.co.jp
Discussion: https://postgr.es/m/10208.1502465077@sss.pgh.pa.us