Verifying variable names in pgbench

We can define variables with any names in pgbench,
but only can refer them with names that consist of [A-Za-z0-9_]+ .
It could cause confusion discussed here:
http://archives.postgresql.org/message-id/4B272833.8080500@2ndquadrant.com

The attached patch verifies variable names at definition.
    $ pgbench -D var:name=value
    (global): invalid variable name 'var:name'

It would help users to determine why their custom scripts failed
when they misuse "\setXXX :varname" (the colon should be removed).

Regards,
---
Takahiro Itagaki
NTT Open Source Software Center