When pg_dump prompts the user for a password, it remembers the password
for possible re-use by parallel worker processes.  However, libpq might
have extracted the password from a connection string originally passed
as "dbname".  Since we don't record the original form of dbname but
break it down to host/port/etc, the password gets lost.  Fix that by
retrieving the actual password from the PGconn.

(It strikes me that this whole approach is rather broken, as it will also
lose other information such as options that might have been present in
the connection string.  But we'll leave that problem for another day.)

In passing, get rid of rather silly use of malloc() for small fixed-size
arrays.

Back-patch to 9.3 where parallel pg_dump was introduced.

Report and fix by Zeus Kronion, adjusted a bit by Michael Paquier and me