WIP: Fix parallel workers connection bug in pg_dump (Bug #13727)

Parallel workers were failing to connect to the database when running
pg_dump with a connection string. The first of the following two commands
runs without errors, while the second one fails:
pg_dump "postgres://my-user:my-password(at)my(dot)hostname(dot)com:5432/my-db" -Fd -f
my-dump
pg_dump "postgres://my-user:my-password(at)my(dot)hostname(dot)com:5432/my-db" -Fd
--jobs=9 -f my-dump

The error message:
pg_dump: [parallel archiver] connection to database "my-db" failed:
fe_sendauth: no password supplied

The password is not being stored correctly in the PGconn object when
connecting with a connection string.

This is my first time contributing to Postgres, so I tried to stick to the
instructions from the "Submitting a Patch" wiki. This submission is for
discussion because I haven't figured out how to write regression tests for
this patch yet (and I would appreciate guidance).

Target branch: master
Compiles and tests successfully: true
Platform-specific items: none
Regression tests: still needed
Documentation: N/A
Performance implications: none