BUG #15012: psql doesn't use server's default port

The following bug has been logged on the website:

Bug reference:      15012
Logged by:          Константин Калинин
Email address:      konstantin@kkalinin.pro
PostgreSQL version: 10.1
Operating system:   Xubuntu 16.04
Description:

I had PosgreSQL 9.6 server, that ran on 5432 port and installed 10.1
simultaneously so it started to use 5433 port. 
Both were installed via apt repo http://apt.postgresql.org/pub/repos/apt/
xenial-pgdg main.
After that I've uninstalled 9.6.

Now, though `psql` says in help that it's going to connect to 5433 port by
default, it's connecting to 5432. So I have to explicitly set a `-p 5433`
option.