There are two cases when libpq may search for "localhost":
1. When host in the connection string is not specified and it is using default socket directory path.
2. When specified host matches default socket directory path.
Since we don't know the value of default socket directory path and effectively can't detect the case 2, the best strategy to mitigate the problem would be to add "localhost" if we detected a "host" be a unix socket directory (it starts with '/' character).
Close#3134
