pgbench -M option can be specified more than once

While playing with pgbench, I found multiple "-M query_mode" can be
set more than once. For example,

$ pgbench -p 11002 -M extended -S -M prepared test
starting vacuum...end.
transaction type: <builtin: select only>
scaling factor: 1
query mode: prepared
number of clients: 1
number of threads: 1
number of transactions per client: 10
number of transactions actually processed: 10/10
latency average = 0.490 ms
tps = 2040.372858 (including connections establishing)
tps = 3920.541599 (excluding connections establishing)

In this case the last "-M prepared" was chosen as the query mode. This
is sloppy because we cannot actually choose different -M at the same
time.

Attached is a patch to detect such an error.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp