postgresql: improve init script

postgresql: improve init script

Use newly introduced procd_add_restart_mount_trigger to make sure
postmaster gets started only once PGDATA becomes available.
Relocate socket directory to /var/lib/postgresql to make it possible
to run postgresql inside a ujail.
Use signal for shutdown, so it works nicely with jail.
Allow multiple script in UCI 'config postgres-db' to be a list and
run them in order listed.
User more silent methods to check for db or role existence and make
it easy to create several databases owned by the same user by passing
the same credentials multiple times.
Remove disfunctional reload handler.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>