Warn about initdb using mount-points

Add code to detect and warn about trying to initdb or create pg_xlog on
mount points.
Add ALTER ROLE ALL SET command

This generalizes the existing ALTER ROLE ... SET and ALTER DATABASE
... SET functionality to allow creating settings that apply to all users
in all databases.

reviewed by Pavel Stehule
Merge branch 'master' of /home/kgrittn/pg/postgresql into matview