postgresql post-install instructions are unreliable

[weior@…]

During install, this is displayed:

To create a database instance, after install do
 sudo mkdir -p /opt/local/var/db/postgresql91/defaultdb
 sudo chown postgres:postgres /opt/local/var/db/postgresql91/defaultdb
 sudo su postgres -c '/opt/local/lib/postgresql91/bin/initdb -D /opt/local/var/db/postgresql91/defaultdb'

However, the last command with su is sensitive to current directory, and usually it will fail with confusing message:

shell-init: error retrieving current directory: getcwd: cannot access parent directories: Permission denied

I suggest adding cd to these instructions.

[blb@…]

Switching them over to use notes instead of ui_msg would also be a good change...

[ryandesign@…]

Duplicate of #14706.