postgresql: update to version 9.5.4 and major rework

postgresql: update to version 9.5.4 and major rework

 * convert package build to use host-build for ecpg, pg_config and zic
 * introduce /lib/functions/postgresql.sh to be used by packages
   requiring a postgres database to exist as well as postgres' init
 * no longer require shadow-su, patch pg_ctl to setuid() ifself instead
 * auto-create database directory if there is enough free space
 * auto-create databases configured in UCI
 * remove some dead uci config options
 * grab maintainership

Signed-off-by: Daniel Golle <redacted>