pgsql: Prevent evaluation of backticks while discarding unwanted

Log Message:
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Prevent evaluation of backticks while discarding unwanted arguments
after an unknown or failed psql backslash command, and also while
discarding "extra" arguments of a putatively valid backslash command.
In the case of an unknown/failed command, make sure we discard the
whole rest of the line, rather than trying to resume at the next
backslash. Per discussion with Thomer Gil.

Modified Files:
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pgsql/src/bin/psql:
command.c (r1.137 -> r1.138)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/command.c.diff?r1=1.137&r2=1.138)
psqlscan.h (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/psqlscan.h.diff?r1=1.3&r2=1.4)
psqlscan.l (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/psqlscan.l.diff?r1=1.7&r2=1.8)