[bug fix] PG10: libpq doesn't connect to alternative hosts when some errors occur

Hello,

I found a problem with libpq connection failover. When libpq cannot connect to earlier hosts in the host list, it doesn't try to connect to other hosts. For example, when you specify a wrong port that some non-postgres program is using, or some non-postgres program is using PG's port unexpectedly, you get an error like this:

$ psql -h localhost -p 23
psql: received invalid response to SSL negotiation: 
$ psql -h localhost -p 23 -d "sslmode=disable"
psql: expected authentication request from server, but received 

Likewise, when the first host has already reached max_connections, libpq doesn't attempt the connection aginst later hosts.

The attached patch fixes this. I'll add this item in the PostgreSQL 10 Open Items.

Regards
Takayuki Tsunakawa