remove dbname arg from bootstrap mode

This little patch removes the dbname argument to InitPostgres in the
bootstrap mode.  It also cleans a bit of fallout: initdb uselessly
passing an unused database name, postmaster starting a dummy process,
and removes the -y option to the bootstrap process, which is
undocumented and useless.

Unless there are objections I intend to apply this shortly.
AFAICS it doesn't need a catversion bump.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.