Due to simplistic quoting and confusion of database names with conninfo
strings, roles with the CREATEDB or CREATEROLE option could escalate to
superuser privileges when a superuser next ran certain maintenance
commands.  The new coding rule for PQconnectdbParams() calls, documented
at conninfo_array_parse(), is to pass expand_dbname=true and wrap
literal database names in a trivial connection string.  Escape
zero-length values in appendConnStrVal().  Back-patch to 9.1 (all
supported versions).

Nathan Bossart, Michael Paquier, and Noah Misch.  Reviewed by Peter
Eisentraut.  Reported by Nathan Bossart.

Security: CVE-2016-5424