initdb: Refactor PG_CMD_PUTS loops

Keeping the SQL commands that initdb runs in string arrays before
feeding them to PG_CMD_PUTS() seems unnecessarily verbose and
inflexible.  In some cases, the array only has one member.  In other
cases, one might want to use PG_CMD_PRINTF() instead, to parametrize a
command, but that would require breaking up the loop or using
workarounds like replace_token().  This patch unwinds all that; it's 
much simpler that way.