pgbench - use enum for meta commands

Minor code enhancement.

While having a look at adding if/elif/else/endif to pgbench, and given the 
current gset/cset added meta commands in cf queue, it occured to me that 
repeated string comparisons to check for the various meta commands is 
neither efficient nor readable. Use an enum instead, which are extensively 
used already for other similar purposes.

-- 
Fabien.