[FIX] db dump: report errors due to missing password + pass PGPASSWORD even on Unix

[FIX] db dump: report errors due to missing password + pass PGPASSWORD even on Unix

Even on Unix systems the PGPASSWORD env var
is necessary when Postgres is configured to
deny unix socket connections or to require
a password anyway. This avoids asking admins
to manually create a ~/.pgpass file in order
to use the dump/restore tools.
Also did some cleanup to ensure the PGPASSWORD
var is removed from the environment as soon
as the operation is done.

lp bug: https://launchpad.net/bugs/790164 fixed
lp bug: https://launchpad.net/bugs/919100 fixed

bzr revid: odo@openerp.com-20120131110849-tji6ipjovxc9oi4v