Fix bash 4 unbound variable in `bash -c`, fix UNIX socket with psql and localhost

[Nastradamus]

If we can't connect to local ssh, we fail with an error in Bash 4:

dmius@test:~/postgres-health-check$ ./check -p 5432 -h localhost --username test --project test --dbname test -e 1
./check: line 344: pgpas_subst: unbound variable

Also, psql must connect to localhost via UNIX socket.