Second pass over run-time configuration system. Adjust priorities on some

Second pass over run-time configuration system. Adjust priorities on some
option settings. Sort out SIGHUP vs BACKEND -- there is no total ordering
here, so make explicit checks. Add comments explaining all of this.
Removed permissions check on SHOW command.

Add examine_subclass to the game, rename to SQL_inheritance to fit the
official data model better. Adjust documentation.

Standalone backend needs to reset all options before it starts. To
facilitate that, have IsUnderPostmaster be set by the postmaster itself,
don't wait for the magic -p switch.

Also make sure that all environment variables and argv's survive
init_ps_display(). Use strdup where necessary.

Have initdb make configuration files (postgresql.conf, pg_hba.conf) mode
0600 -- having configuration files is no fun if you can't edit them.