Allow vacuumdb to only analyze

One of the talks at PGCon (update in place?) recommended running  
vacuumdb -z to analyze all tables to rebuild statistics. Problem with  
that is it also vacuums everything. ISTM it'd be useful to be able to  
just vacuum all databases in a cluster, so I hacked it into vacuumdb.

Of course, using a command called vacuumdb is rather silly, but I  
don't see a reasonable way to deal with that. I did change the name  
of the functions from vacuum_* to process_*, since they can vacuum  
and/or analyze.

The only thing I see missing is the checks for invalid combinations  
of options, which I'm thinking should go in the function rather than  
in the option parsing section. But I didn't want to put any more  
effort into this if it's not something we actually want.