Refactor code of reindexdb for query generation

 Refactor code of reindexdb for query generation
 
 This merges the portion related to REINDEX SYSTEM into the routine
 already available for all the other reindex types, making the query
 generation cleaner.  While on it, change the handling of the reindex
 types using an enum, which allows to get rid of the hardcoded strings
 used directly in the query generation present for the same purpose (aka
 "TABLE", "DATABASE", etc.).
 
 Per discussion with Julien Rouhaud, Tom Lane, Alvaro Herrera and me.
 
 Author: Julien Rouhaud
 Discussion: https://postgr.es/m/CAOBaU_bSmSik_WRK9niDnm-3NkNZky6+uKxkmQwvthZvMWpS5A@mail.gmail.com