In arguments, these meta-commands wrongly treated each pair as closing
the double quoted string.  Make the behavior match the documentation.
This is a compatibility break, but I more expect to find software with
untested reliance on the documented behavior than software reliant on
today's behavior.  Back-patch to 9.1 (all supported versions).

Reviewed by Tom Lane and Peter Eisentraut.

Security: CVE-2016-5424